add nunique GH9548

Author: eshort0401

doc/api/dataarray.rst

@@ -162,6 +162,7 @@ Aggregation
    DataArray.min
    DataArray.mean
    DataArray.median
+   DataArray.nunique
    DataArray.prod
    DataArray.sum
    DataArray.std

doc/api/dataset.rst

@@ -169,6 +169,7 @@ Aggregation
    Dataset.min
    Dataset.mean
    Dataset.median
+   Dataset.nunique
    Dataset.prod
    Dataset.sum
    Dataset.std

doc/api/datatree.rst

@@ -266,6 +266,7 @@ Aggregate data in all nodes in the subtree simultaneously.
    DataTree.min
    DataTree.mean
    DataTree.median
+   DataTree.nunique
    DataTree.prod
    DataTree.sum
    DataTree.std

doc/whats-new.rst

@@ -14,6 +14,13 @@ v2025.12.1 (unreleased)
 New Features
 ~~~~~~~~~~~~
 
+- :py:func:`combine_nested` now support :py:class:`DataTree` objects
+  (:pull:`10849`).
+  By `Stephan Hoyer <https://github.com/shoyer>`_.
+- Add :py:func:`nunique` reduction function (:issue:`9548`), which behaves like
+  :py:func:`pandas.DataFrame.nunique` applied along specific dimensions.
+  By `Ewan Short <https://github.com/eshort0401>`_.
+
 
 Breaking Changes
 ~~~~~~~~~~~~~~~~