From a6410a2a5bc7dc6e2704014b9e9534229350a90b Mon Sep 17 00:00:00 2001
From: r3kste <138380708+r3kste@users.noreply.github.com>
Date: Wed, 3 Dec 2025 21:07:31 +0530
Subject: [PATCH 1/4] Implementation of `altair` backend.

---
 .tools/envs/testenv-linux.yml                 |   1 +
 .tools/envs/testenv-nevergrad.yml             |   1 +
 .tools/envs/testenv-numpy.yml                 |   1 +
 .tools/envs/testenv-others.yml                |   1 +
 .tools/envs/testenv-pandas.yml                |   1 +
 .tools/envs/testenv-plotly.yml                |   1 +
 docs/rtd_environment.yml                      |   1 +
 .../how_to_change_plotting_backend.ipynb      |  33 +++
 environment.yml                               |   1 +
 src/optimagic/config.py                       |   1 +
 src/optimagic/visualization/backends.py       | 241 +++++++++++++++++-
 .../visualization/convergence_plot.py         |   3 +-
 src/optimagic/visualization/history_plots.py  |   7 +-
 src/optimagic/visualization/profile_plot.py   |   3 +-
 src/optimagic/visualization/slice_plot.py     |   2 +-
 .../visualization/test_convergence_plot.py    |   1 +
 .../visualization/test_profile_plot.py        |   1 +
 17 files changed, 290 insertions(+), 10 deletions(-)

diff --git a/.tools/envs/testenv-linux.yml b/.tools/envs/testenv-linux.yml
index 87e08d82b..7fd4af3f3 100644
--- a/.tools/envs/testenv-linux.yml
+++ b/.tools/envs/testenv-linux.yml
@@ -21,6 +21,7 @@ dependencies:
   - plotly>=6.2  # run, tests
   - matplotlib  # tests
   - bokeh  # tests
+  - altair  # tests
   - pybaum>=0.1.2  # run, tests
   - scipy>=1.2.1  # run, tests
   - sqlalchemy  # run, tests
diff --git a/.tools/envs/testenv-nevergrad.yml b/.tools/envs/testenv-nevergrad.yml
index d369e0c55..d02785f4d 100644
--- a/.tools/envs/testenv-nevergrad.yml
+++ b/.tools/envs/testenv-nevergrad.yml
@@ -19,6 +19,7 @@ dependencies:
   - plotly>=6.2  # run, tests
   - matplotlib  # tests
   - bokeh  # tests
+  - altair  # tests
   - pybaum>=0.1.2  # run, tests
   - scipy>=1.2.1  # run, tests
   - sqlalchemy  # run, tests
diff --git a/.tools/envs/testenv-numpy.yml b/.tools/envs/testenv-numpy.yml
index ed78d560c..33b4f151b 100644
--- a/.tools/envs/testenv-numpy.yml
+++ b/.tools/envs/testenv-numpy.yml
@@ -19,6 +19,7 @@ dependencies:
   - plotly>=6.2  # run, tests
   - matplotlib  # tests
   - bokeh  # tests
+  - altair  # tests
   - pybaum>=0.1.2  # run, tests
   - scipy>=1.2.1  # run, tests
   - sqlalchemy  # run, tests
diff --git a/.tools/envs/testenv-others.yml b/.tools/envs/testenv-others.yml
index 5b8a953cc..7fe42abbc 100644
--- a/.tools/envs/testenv-others.yml
+++ b/.tools/envs/testenv-others.yml
@@ -19,6 +19,7 @@ dependencies:
   - plotly>=6.2  # run, tests
   - matplotlib  # tests
   - bokeh  # tests
+  - altair  # tests
   - pybaum>=0.1.2  # run, tests
   - scipy>=1.2.1  # run, tests
   - sqlalchemy  # run, tests
diff --git a/.tools/envs/testenv-pandas.yml b/.tools/envs/testenv-pandas.yml
index a0418ff9c..d62234ede 100644
--- a/.tools/envs/testenv-pandas.yml
+++ b/.tools/envs/testenv-pandas.yml
@@ -19,6 +19,7 @@ dependencies:
   - plotly>=6.2  # run, tests
   - matplotlib  # tests
   - bokeh  # tests
+  - altair  # tests
   - pybaum>=0.1.2  # run, tests
   - scipy>=1.2.1  # run, tests
   - sqlalchemy  # run, tests
diff --git a/.tools/envs/testenv-plotly.yml b/.tools/envs/testenv-plotly.yml
index abf4dd211..41e8dc938 100644
--- a/.tools/envs/testenv-plotly.yml
+++ b/.tools/envs/testenv-plotly.yml
@@ -19,6 +19,7 @@ dependencies:
   - pandas  # run, tests
   - matplotlib  # tests
   - bokeh  # tests
+  - altair  # tests
   - pybaum>=0.1.2  # run, tests
   - scipy>=1.2.1  # run, tests
   - sqlalchemy  # run, tests
diff --git a/docs/rtd_environment.yml b/docs/rtd_environment.yml
index 5ff33f75c..40e0566a5 100644
--- a/docs/rtd_environment.yml
+++ b/docs/rtd_environment.yml
@@ -20,6 +20,7 @@ dependencies:
   - pybaum
   - matplotlib
   - bokeh
+  - altair
   - seaborn
   - numpy
   - pandas
diff --git a/docs/source/how_to/how_to_change_plotting_backend.ipynb b/docs/source/how_to/how_to_change_plotting_backend.ipynb
index c8e07237f..3470c31d3 100644
--- a/docs/source/how_to/how_to_change_plotting_backend.ipynb
+++ b/docs/source/how_to/how_to_change_plotting_backend.ipynb
@@ -57,6 +57,20 @@
     "```{warning}\n",
     "Bokeh applies themes globally. Passing the `template` parameter to a plotting function updates the theme for all existing and future Bokeh plots. If you do not pass `template`, a default template is applied, which will also change the global theme.\n",
     "```\n",
+    "\n",
+    ":::\n",
+    "\n",
+    ":::{tab-item} Altair\n",
+    "To select the Altair backend, set `backend=\"altair\"`.\n",
+    "\n",
+    "The returned figure object is an [`altair.Chart`](https://altair-viz.github.io/user_guide/generated/toplevel/altair.Chart.html).\n",
+    "\n",
+    "```{note}\n",
+    "In case of grid plots (such as `convergence_plot` or `slice_plot`), the returned object is an [`altair.VConcatChart`](https://altair-viz.github.io/user_guide/generated/toplevel/altair.VConcatChart.html).\n",
+    "```\n",
+    "\n",
+    ":::\n",
+    "\n",
     "::::"
    ]
   },
@@ -170,6 +184,25 @@
     "p = om.criterion_plot(results, backend=\"bokeh\")\n",
     "show(p)"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "12",
+   "metadata": {},
+   "source": [
+    "## Altair"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "13",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "chart = om.criterion_plot(results, backend=\"altair\")\n",
+    "chart.show()"
+   ]
   }
  ],
  "metadata": {
diff --git a/environment.yml b/environment.yml
index d62a675b0..642da8ea8 100644
--- a/environment.yml
+++ b/environment.yml
@@ -23,6 +23,7 @@ dependencies:
   - plotly>=6.2  # run, tests
   - matplotlib  # tests
   - bokeh  # tests
+  - altair  # tests
   - pybaum>=0.1.2  # run, tests
   - scipy>=1.2.1  # run, tests
   - sqlalchemy  # run, tests
diff --git a/src/optimagic/config.py b/src/optimagic/config.py
index def8302f7..2f944bd9c 100644
--- a/src/optimagic/config.py
+++ b/src/optimagic/config.py
@@ -63,6 +63,7 @@ def _is_installed(module_name: str) -> bool:
 
 IS_MATPLOTLIB_INSTALLED = _is_installed("matplotlib")
 IS_BOKEH_INSTALLED = _is_installed("bokeh")
+IS_ALTAIR_INSTALLED = _is_installed("altair")
 
 # ======================================================================================
 # Check if pandas version is newer or equal to version 2.1.0
diff --git a/src/optimagic/visualization/backends.py b/src/optimagic/visualization/backends.py
index c8195cc55..54753adb7 100644
--- a/src/optimagic/visualization/backends.py
+++ b/src/optimagic/visualization/backends.py
@@ -4,11 +4,16 @@
 import numpy as np
 import plotly.graph_objects as go
 
-from optimagic.config import IS_BOKEH_INSTALLED, IS_MATPLOTLIB_INSTALLED
+from optimagic.config import (
+    IS_ALTAIR_INSTALLED,
+    IS_BOKEH_INSTALLED,
+    IS_MATPLOTLIB_INSTALLED,
+)
 from optimagic.exceptions import InvalidPlottingBackendError, NotInstalledError
 from optimagic.visualization.plotting_utilities import LineData, MarkerData
 
 if TYPE_CHECKING:
+    import altair as alt
     import bokeh
     import matplotlib.pyplot as plt
 
@@ -592,9 +597,228 @@ def _grid_line_plot_bokeh(
     return grid
 
 
+def _line_plot_altair(
+    lines: list[LineData],
+    *,
+    title: str | None,
+    xlabel: str | None,
+    xrange: tuple[float, float] | None,
+    ylabel: str | None,
+    yrange: tuple[float, float] | None,
+    template: str | None,
+    height: int | None,
+    width: int | None,
+    legend_properties: dict[str, Any] | None,
+    margin_properties: dict[str, Any] | None,
+    horizontal_line: float | None,
+    marker: MarkerData | None,
+    subplot: "alt.Chart | None" = None,
+) -> "alt.Chart":
+    """Create a line plot using Altair.
+
+    Args:
+        ...: All other argument descriptions can be found in the docstring of the
+            `line_plot` function.
+        subplot: An Altair Chart object to which the lines should be plotted.
+            If provided, the plot is drawn on the given Chart. If not provided,
+            a new Chart is created.
+
+    Returns:
+        An Altair Chart object.
+
+    """
+    import altair as alt
+    import pandas as pd
+
+    alt.data_transformers.disable_max_rows()
+
+    if template is None:
+        template = "default"
+    alt.theme.enable("default")
+
+    dfs = []
+    for line in lines:
+        df = pd.DataFrame(
+            {"x": line.x, "y": line.y, "name": line.name, "color": line.color}
+        )
+        dfs.append(df)
+    source = pd.concat(dfs)
+
+    figure_properties: dict[str, str | int] = {}
+    if title is not None:
+        figure_properties["title"] = title
+    if width is not None:
+        figure_properties["width"] = width
+    if height is not None:
+        figure_properties["height"] = height
+
+    chart = (
+        alt.Chart(source)
+        .mark_line()
+        .encode(
+            x=alt.X(
+                "x",
+                title=xlabel.split("{linebreak}") if xlabel else None,
+                scale=alt.Scale(domain=list(xrange)) if xrange else alt.Undefined,
+            ),
+            y=alt.Y(
+                "y",
+                title=ylabel.split("{linebreak}") if ylabel else None,
+                scale=alt.Scale(domain=list(yrange)) if yrange else alt.Undefined,
+            ),
+            color=alt.Color("color:N", scale=None),
+            detail="name:N",
+        )
+        .properties(**figure_properties)
+    )
+
+    if any(line.show_in_legend for line in lines):
+        legend = (
+            alt.Chart(source)
+            .mark_line()
+            .encode(
+                color=alt.Color(
+                    "name:N",
+                    title=None,
+                    legend=alt.Legend(**(legend_properties or {})),
+                    scale=alt.Scale(
+                        domain=[line.name for line in lines if line.show_in_legend],
+                        range=[
+                            line.color or "" for line in lines if line.show_in_legend
+                        ],
+                    ),
+                )
+            )
+        )
+        chart = chart + legend
+
+    if horizontal_line is not None:
+        hline = (
+            alt.Chart(pd.DataFrame({"y": [horizontal_line]})).mark_rule().encode(y="y")
+        )
+        chart = chart + hline
+
+    if marker is not None:
+        marker_chart = (
+            alt.Chart(pd.DataFrame({"x": [marker.x], "y": [marker.y]}))
+            .mark_point(size=100, shape="circle", color=marker.color, filled=True)
+            .encode(x="x", y="y")
+        )
+        chart = chart + marker_chart
+
+    return chart
+
+
+def _grid_line_plot_altair(
+    lines_list: list[list[LineData]],
+    *,
+    n_rows: int,
+    n_cols: int,
+    titles: list[str] | None,
+    xlabels: list[str] | None,
+    xrange: tuple[float, float] | None,
+    share_x: bool,
+    ylabels: list[str] | None,
+    yrange: tuple[float, float] | None,
+    share_y: bool,
+    template: str | None,
+    height: int | None,
+    width: int | None,
+    legend_properties: dict[str, Any] | None,
+    margin_properties: dict[str, Any] | None,
+    plot_title: str | None,
+    marker_list: list[MarkerData] | None,
+    make_subplot_kwargs: dict[str, Any] | None = None,
+) -> "alt.VConcatChart":
+    """Create a grid of line plots using Altair.
+
+    Args:
+        ...: All other argument descriptions can be found in the docstring of the
+            `grid_line_plot` function.
+
+    Returns:
+        An Altair VConcatChart object.
+
+    """
+    import altair as alt
+
+    subplot_height = height // n_rows if height else None
+    subplot_width = width // n_cols if width else None
+
+    charts = []
+    for row_idx in range(n_rows):
+        chart_row = []
+        for col_idx in range(n_cols):
+            i = row_idx * n_cols + col_idx
+            if i >= len(lines_list):
+                break
+
+            chart = _line_plot_altair(
+                lines_list[i],
+                title=titles[i] if titles else None,
+                xlabel=xlabels[i] if xlabels else None,
+                xrange=xrange,
+                ylabel=ylabels[i] if ylabels else None,
+                yrange=yrange,
+                template=template,
+                height=subplot_height,
+                width=subplot_width,
+                legend_properties=legend_properties,
+                margin_properties=None,
+                horizontal_line=None,
+                marker=marker_list[i] if marker_list else None,
+                subplot=None,
+            )
+
+            chart_row.append(chart)
+        charts.append(chart_row)
+
+    row_selections = [
+        alt.selection_interval(bind="scales", encodings=["y"]) for _ in range(n_rows)
+    ]
+    col_selections = [
+        alt.selection_interval(bind="scales", encodings=["x"]) for _ in range(n_cols)
+    ]
+
+    for row_idx, row in enumerate(charts):
+        for col_idx in range(len(row)):
+            chart = row[col_idx]
+
+            params = []
+            if share_y:
+                # Share y-axis for all subplots in the same row
+                params.append(row_selections[row_idx])
+            else:
+                # Use independent y-axes for each subplot
+                params.append(alt.selection_interval(bind="scales", encodings=["y"]))
+            if share_x:
+                # Share x-axis for all subplots in the same column
+                params.append(col_selections[col_idx])
+            else:
+                # Use independent x-axes for each subplot
+                params.append(alt.selection_interval(bind="scales", encodings=["x"]))
+            chart = chart.add_params(*params)
+
+            if share_y and col_idx > 0:
+                # Hide y-axis ticklabels for all subplots except the leftmost column
+                chart = chart.encode(y=alt.Y(axis=alt.Axis(labels=False)))
+            if share_x and row_idx < n_rows - 1:
+                # Hide x-axis ticklabels for all subplots except the bottom row
+                chart = chart.encode(x=alt.X(axis=alt.Axis(labels=False)))
+
+            charts[row_idx][col_idx] = chart
+
+    grid_chart = alt.vconcat(*[alt.hconcat(*row) for row in charts])
+
+    if plot_title is not None:
+        grid_chart = grid_chart.properties(title=plot_title)
+
+    return grid_chart
+
+
 def line_plot(
     lines: list[LineData],
-    backend: Literal["plotly", "matplotlib", "bokeh"] = "plotly",
+    backend: Literal["plotly", "matplotlib", "bokeh", "altair"] = "plotly",
     *,
     title: str | None = None,
     xlabel: str | None = None,
@@ -657,7 +881,7 @@ def line_plot(
 
 def grid_line_plot(
     lines_list: list[list[LineData]],
-    backend: Literal["plotly", "matplotlib", "bokeh"] = "plotly",
+    backend: Literal["plotly", "matplotlib", "bokeh", "altair"] = "plotly",
     *,
     n_rows: int,
     n_cols: int,
@@ -750,18 +974,25 @@ def grid_line_plot(
         _line_plot_bokeh,
         _grid_line_plot_bokeh,
     ),
+    "altair": (
+        IS_ALTAIR_INSTALLED,
+        _line_plot_altair,
+        _grid_line_plot_altair,
+    ),
 }
 
 
 @overload
 def _get_plot_function(
-    backend: Literal["plotly", "matplotlib", "bokeh"], grid_plot: Literal[False]
+    backend: Literal["plotly", "matplotlib", "bokeh", "altair"],
+    grid_plot: Literal[False],
 ) -> LinePlotFunction: ...
 
 
 @overload
 def _get_plot_function(
-    backend: Literal["plotly", "matplotlib", "bokeh"], grid_plot: Literal[True]
+    backend: Literal["plotly", "matplotlib", "bokeh", "altair"],
+    grid_plot: Literal[True],
 ) -> GridLinePlotFunction: ...
 
 
diff --git a/src/optimagic/visualization/convergence_plot.py b/src/optimagic/visualization/convergence_plot.py
index 3287a9b4d..487749d11 100644
--- a/src/optimagic/visualization/convergence_plot.py
+++ b/src/optimagic/visualization/convergence_plot.py
@@ -19,6 +19,7 @@
         "place": "right",
         "label_text_font_size": "8pt",
     },
+    "altair": {"orient": "right"},
 }
 
 BACKEND_TO_CONVERGENCE_PLOT_MARGIN_PROPERTIES: dict[str, dict[str, int]] = {
@@ -75,7 +76,7 @@ def convergence_plot(
     x_precision: float = 1e-4,
     y_precision: float = 1e-4,
     combine_plots_in_grid: bool = True,
-    backend: Literal["plotly", "matplotlib", "bokeh"] = "plotly",
+    backend: Literal["plotly", "matplotlib", "bokeh", "altair"] = "plotly",
     template: str | None = None,
     palette: list[str] | str = DEFAULT_PALETTE,
 ) -> Any:
diff --git a/src/optimagic/visualization/history_plots.py b/src/optimagic/visualization/history_plots.py
index d14d8412b..14d684508 100644
--- a/src/optimagic/visualization/history_plots.py
+++ b/src/optimagic/visualization/history_plots.py
@@ -30,6 +30,9 @@
     "bokeh": {
         "location": "top_right",
     },
+    "altair": {
+        "orient": "top-right",
+    },
 }
 
 
@@ -40,7 +43,7 @@ def criterion_plot(
     results: ResultOrPath | list[ResultOrPath] | dict[str, ResultOrPath],
     names: list[str] | str | None = None,
     max_evaluations: int | None = None,
-    backend: Literal["plotly", "matplotlib", "bokeh"] = "plotly",
+    backend: Literal["plotly", "matplotlib", "bokeh", "altair"] = "plotly",
     template: str | None = None,
     palette: list[str] | str = DEFAULT_PALETTE,
     stack_multistart: bool = False,
@@ -158,7 +161,7 @@ def params_plot(
     result: ResultOrPath,
     selector: Callable[[PyTree], PyTree] | None = None,
     max_evaluations: int | None = None,
-    backend: Literal["plotly", "matplotlib", "bokeh"] = "plotly",
+    backend: Literal["plotly", "matplotlib", "bokeh", "altair"] = "plotly",
     template: str | None = None,
     palette: list[str] | str = DEFAULT_PALETTE,
     show_exploration: bool = False,
diff --git a/src/optimagic/visualization/profile_plot.py b/src/optimagic/visualization/profile_plot.py
index cf9482aba..beda49554 100644
--- a/src/optimagic/visualization/profile_plot.py
+++ b/src/optimagic/visualization/profile_plot.py
@@ -25,6 +25,7 @@
         "label_text_font_size": "8pt",
         "title": "algorithm",
     },
+    "altair": {"orient": "top-right", "title": "algorithm"},
 }
 
 BACKEND_TO_PROFILE_PLOT_MARGIN_PROPERTIES: dict[str, dict[str, Any]] = {
@@ -44,7 +45,7 @@ def profile_plot(
     stopping_criterion: Literal["x", "y", "x_and_y", "x_or_y"] = "y",
     x_precision: float = 1e-4,
     y_precision: float = 1e-4,
-    backend: Literal["plotly", "matplotlib", "bokeh"] = "plotly",
+    backend: Literal["plotly", "matplotlib", "bokeh", "altair"] = "plotly",
     template: str | None = None,
     palette: list[str] | str = DEFAULT_PALETTE,
 ) -> Any:
diff --git a/src/optimagic/visualization/slice_plot.py b/src/optimagic/visualization/slice_plot.py
index 206f6a8ec..f831b9b2d 100644
--- a/src/optimagic/visualization/slice_plot.py
+++ b/src/optimagic/visualization/slice_plot.py
@@ -44,7 +44,7 @@ def slice_plot(
     share_y: bool = True,
     expand_yrange: float = 0.02,
     share_x: bool = False,
-    backend: Literal["plotly", "matplotlib", "bokeh"] = "plotly",
+    backend: Literal["plotly", "matplotlib", "bokeh", "altair"] = "plotly",
     template: str | None = None,
     color: str | None = DEFAULT_PALETTE[0],
     title: str | None = None,
diff --git a/tests/optimagic/visualization/test_convergence_plot.py b/tests/optimagic/visualization/test_convergence_plot.py
index 469503bf7..3697425b9 100644
--- a/tests/optimagic/visualization/test_convergence_plot.py
+++ b/tests/optimagic/visualization/test_convergence_plot.py
@@ -54,6 +54,7 @@ def test_convergence_plot_default_options(benchmark_results):
     {"y_precision": 1e-5},
     {"backend": "matplotlib"},
     {"backend": "bokeh"},
+    {"backend": "altair"},
 ]
 
 
diff --git a/tests/optimagic/visualization/test_profile_plot.py b/tests/optimagic/visualization/test_profile_plot.py
index c8fd1817c..43f6310af 100644
--- a/tests/optimagic/visualization/test_profile_plot.py
+++ b/tests/optimagic/visualization/test_profile_plot.py
@@ -182,6 +182,7 @@ def test_extract_profile_plot_lines():
     {"stopping_criterion": "x_or_y"},
     {"backend": "matplotlib"},
     {"backend": "bokeh"},
+    {"backend": "altair"},
 ]
 
 

From 2097b3659e5cfa6bee63f67c7e69af0d2ff01209 Mon Sep 17 00:00:00 2001
From: r3kste <138380708+r3kste@users.noreply.github.com>
Date: Fri, 5 Dec 2025 15:02:43 +0530
Subject: [PATCH 2/4] Fix Altair chart concatenation and stubs

---
 .../how_to_change_plotting_backend.ipynb      |  6 +--
 src/optimagic/optimization/history.py         |  2 +-
 src/optimagic/visualization/backends.py       | 46 ++++++++++++++++---
 src/optimagic/visualization/slice_plot.py     |  2 +-
 4 files changed, 44 insertions(+), 12 deletions(-)

diff --git a/docs/source/how_to/how_to_change_plotting_backend.ipynb b/docs/source/how_to/how_to_change_plotting_backend.ipynb
index 3470c31d3..75523f532 100644
--- a/docs/source/how_to/how_to_change_plotting_backend.ipynb
+++ b/docs/source/how_to/how_to_change_plotting_backend.ipynb
@@ -66,7 +66,7 @@
     "The returned figure object is an [`altair.Chart`](https://altair-viz.github.io/user_guide/generated/toplevel/altair.Chart.html).\n",
     "\n",
     "```{note}\n",
-    "In case of grid plots (such as `convergence_plot` or `slice_plot`), the returned object is an [`altair.VConcatChart`](https://altair-viz.github.io/user_guide/generated/toplevel/altair.VConcatChart.html).\n",
+    "In case of grid plots (such as `convergence_plot` or `slice_plot`), the returned object is either an [`altair.Chart`](https://altair-viz.github.io/user_guide/generated/toplevel/altair.Chart.html) if there is only one subplot, an [`altair.HConcatChart`](https://altair-viz.github.io/user_guide/generated/toplevel/altair.HConcatChart.html) if there is only one row, or an [`altair.VConcatChart`](https://altair-viz.github.io/user_guide/generated/toplevel/altair.VConcatChart.html) otherwise.\n",
     "```\n",
     "\n",
     ":::\n",
@@ -207,7 +207,7 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 3",
+   "display_name": "optimagic",
    "language": "python",
    "name": "python3"
   },
@@ -221,7 +221,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.10.18"
+   "version": "3.10.17"
   }
  },
  "nbformat": 4,
diff --git a/src/optimagic/optimization/history.py b/src/optimagic/optimization/history.py
index 966316826..2ce0c6cd7 100644
--- a/src/optimagic/optimization/history.py
+++ b/src/optimagic/optimization/history.py
@@ -207,7 +207,7 @@ def params_data(
         """
         wide = pd.DataFrame(self.flat_params, columns=self.flat_param_names)
         wide["task"] = _task_to_categorical(self.task)
-        wide["fun"] = self.fun
+        wide["fun"] = self.fun  # type: ignore[assignment]
 
         # If requested, we collapse the batches and only keep the parameters that led to
         # the minimal (or maximal) function value in each batch.
diff --git a/src/optimagic/visualization/backends.py b/src/optimagic/visualization/backends.py
index 54753adb7..efb60bd3f 100644
--- a/src/optimagic/visualization/backends.py
+++ b/src/optimagic/visualization/backends.py
@@ -729,7 +729,7 @@ def _grid_line_plot_altair(
     plot_title: str | None,
     marker_list: list[MarkerData] | None,
     make_subplot_kwargs: dict[str, Any] | None = None,
-) -> "alt.VConcatChart":
+) -> "alt.Chart | alt.HConcatChart | alt.VConcatChart":
     """Create a grid of line plots using Altair.
 
     Args:
@@ -737,7 +737,8 @@ def _grid_line_plot_altair(
             `grid_line_plot` function.
 
     Returns:
-        An Altair VConcatChart object.
+        An Altair Chart if the grid contains only one subplot, an Altair HConcatChart
+            if 'n_rows' is 1, otherwise an Altair VConcatChart.
 
     """
     import altair as alt
@@ -774,10 +775,16 @@ def _grid_line_plot_altair(
         charts.append(chart_row)
 
     row_selections = [
-        alt.selection_interval(bind="scales", encodings=["y"]) for _ in range(n_rows)
+        alt.selection_interval(
+            bind="scales", encodings=["y"], name=f"share_y_row{row_idx}"
+        )
+        for row_idx in range(n_rows)
     ]
     col_selections = [
-        alt.selection_interval(bind="scales", encodings=["x"]) for _ in range(n_cols)
+        alt.selection_interval(
+            bind="scales", encodings=["x"], name=f"share_x_col{col_idx}"
+        )
+        for col_idx in range(n_cols)
     ]
 
     for row_idx, row in enumerate(charts):
@@ -790,13 +797,25 @@ def _grid_line_plot_altair(
                 params.append(row_selections[row_idx])
             else:
                 # Use independent y-axes for each subplot
-                params.append(alt.selection_interval(bind="scales", encodings=["y"]))
+                params.append(
+                    alt.selection_interval(
+                        bind="scales",
+                        encodings=["x", "y"],
+                        name=f"ind_y_row{row_idx}_col{col_idx}",
+                    )
+                )
             if share_x:
                 # Share x-axis for all subplots in the same column
                 params.append(col_selections[col_idx])
             else:
                 # Use independent x-axes for each subplot
-                params.append(alt.selection_interval(bind="scales", encodings=["x"]))
+                params.append(
+                    alt.selection_interval(
+                        bind="scales",
+                        encodings=["x", "y"],
+                        name=f"ind_x_row{row_idx}_col{col_idx}",
+                    )
+                )
             chart = chart.add_params(*params)
 
             if share_y and col_idx > 0:
@@ -808,7 +827,20 @@ def _grid_line_plot_altair(
 
             charts[row_idx][col_idx] = chart
 
-    grid_chart = alt.vconcat(*[alt.hconcat(*row) for row in charts])
+    row_charts = []
+    for row in charts:
+        row_chart: alt.Chart | alt.HConcatChart
+        if len(row) == 1:
+            row_chart = row[0]
+        else:
+            row_chart = alt.hconcat(*row)
+        row_charts.append(row_chart)
+
+    grid_chart: alt.Chart | alt.HConcatChart | alt.VConcatChart
+    if len(row_charts) == 1:
+        grid_chart = row_charts[0]
+    else:
+        grid_chart = alt.vconcat(*row_charts)
 
     if plot_title is not None:
         grid_chart = grid_chart.properties(title=plot_title)
diff --git a/src/optimagic/visualization/slice_plot.py b/src/optimagic/visualization/slice_plot.py
index f831b9b2d..df98f363a 100644
--- a/src/optimagic/visualization/slice_plot.py
+++ b/src/optimagic/visualization/slice_plot.py
@@ -292,7 +292,7 @@ def _get_plot_data(
         metadata.append(meta)
 
     plot_data = pd.DataFrame(metadata)
-    plot_data["Function Value"] = func_values
+    plot_data["Function Value"] = func_values  # type: ignore[assignment]
 
     return plot_data, internal_params
 

From 5535775e83b635a697c0fd4a06d1271fba93a11f Mon Sep 17 00:00:00 2001
From: r3kste <138380708+r3kste@users.noreply.github.com>
Date: Wed, 10 Dec 2025 03:27:52 +0530
Subject: [PATCH 3/4] set altair renderer as 'jupyter' and add `anywidget` as
 dependency for docs

---
 docs/rtd_environment.yml                      |  1 +
 .../how_to_change_plotting_backend.ipynb      | 23 ++++++++++++++++++-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/docs/rtd_environment.yml b/docs/rtd_environment.yml
index 40e0566a5..ef417f966 100644
--- a/docs/rtd_environment.yml
+++ b/docs/rtd_environment.yml
@@ -21,6 +21,7 @@ dependencies:
   - matplotlib
   - bokeh
   - altair
+  - anywidget
   - seaborn
   - numpy
   - pandas
diff --git a/docs/source/how_to/how_to_change_plotting_backend.ipynb b/docs/source/how_to/how_to_change_plotting_backend.ipynb
index 75523f532..2d3b32f5a 100644
--- a/docs/source/how_to/how_to_change_plotting_backend.ipynb
+++ b/docs/source/how_to/how_to_change_plotting_backend.ipynb
@@ -193,13 +193,34 @@
     "## Altair"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "13",
+   "metadata": {},
+   "source": [
+    ":::{note}\n",
+    "\n",
+    "It is mostly not required to set the renderer manually, as Altair automatically\n",
+    "selects the appropriate renderer based on your environment. In this example,\n",
+    "we explicitly set the renderer to ensure correct display within the documentation.\n",
+    "\n",
+    "Refer to the [Altair documentation](https://altair-viz.github.io/user_guide/display_frontends.html) for more details.\n",
+    "\n",
+    ":::"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "13",
+   "id": "14",
    "metadata": {},
    "outputs": [],
    "source": [
+    "import altair as alt\n",
+    "\n",
+    "# Setting the renderer is mostly not required. See note above.\n",
+    "alt.renderers.enable(\"jupyter\")\n",
+    "\n",
     "chart = om.criterion_plot(results, backend=\"altair\")\n",
     "chart.show()"
    ]

From 7c420b98039386c0cc7e3a062c287e8a0148c8ce Mon Sep 17 00:00:00 2001
From: r3kste <138380708+r3kste@users.noreply.github.com>
Date: Wed, 10 Dec 2025 03:28:42 +0530
Subject: [PATCH 4/4] Make altair charts interactive by default, and minor
 fixes.

---
 src/optimagic/visualization/backends.py     | 6 +++---
 src/optimagic/visualization/profile_plot.py | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/optimagic/visualization/backends.py b/src/optimagic/visualization/backends.py
index efb60bd3f..8299847a2 100644
--- a/src/optimagic/visualization/backends.py
+++ b/src/optimagic/visualization/backends.py
@@ -706,7 +706,7 @@ def _line_plot_altair(
         )
         chart = chart + marker_chart
 
-    return chart
+    return chart.interactive()
 
 
 def _grid_line_plot_altair(
@@ -800,7 +800,7 @@ def _grid_line_plot_altair(
                 params.append(
                     alt.selection_interval(
                         bind="scales",
-                        encodings=["x", "y"],
+                        encodings=["y"],
                         name=f"ind_y_row{row_idx}_col{col_idx}",
                     )
                 )
@@ -812,7 +812,7 @@ def _grid_line_plot_altair(
                 params.append(
                     alt.selection_interval(
                         bind="scales",
-                        encodings=["x", "y"],
+                        encodings=["x"],
                         name=f"ind_x_row{row_idx}_col{col_idx}",
                     )
                 )
diff --git a/src/optimagic/visualization/profile_plot.py b/src/optimagic/visualization/profile_plot.py
index beda49554..003382e5b 100644
--- a/src/optimagic/visualization/profile_plot.py
+++ b/src/optimagic/visualization/profile_plot.py
@@ -25,7 +25,7 @@
         "label_text_font_size": "8pt",
         "title": "algorithm",
     },
-    "altair": {"orient": "top-right", "title": "algorithm"},
+    "altair": {"orient": "right", "title": "algorithm"},
 }
 
 BACKEND_TO_PROFILE_PLOT_MARGIN_PROPERTIES: dict[str, dict[str, Any]] = {