Fix backgroundLayers in waterfallPlot()

Author: MKoroteev-HORIS

docs/examples/jupyter-notebooks/f-4.11.0/waterfall_plot_layers.ipynb

@@ -644,7 +644,7 @@
     "    x = \"Accounts\",\n",
     "    y = \"Values\",\n",
     "    measure = \"Measure\",\n",
-    "    backgroundLayers = listOf(geomPoint(), quarterLayer)\n",
+    "    backgroundLayers = quarterLayer\n",
     ") + \n",
     "    textLayers + \n",
     "    scaleHue(\"paint_a\", guide = \"none\") + \n",

plot-api/src/commonMain/kotlin/org/jetbrains/letsPlot/bistro/waterfall/WaterfallPlot.kt

@@ -110,9 +110,7 @@ package org.jetbrains.letsPlot.bistro.waterfall
  *  - "Score: {.2f}" -> "Score: 12.45"
  *  - "Score: {}" -> "Score: 12.454789"
  * @param backgroundLayers will be added behind the waterfall
- * Background layers to be added to the plot. Accepts either:
- *  - A single layer (e.g., `geomBar { ... }`).
- *  - A list of layers (`List<Layer>`).
+ * Background layers to be added to the plot, a single Layer or FeatureList.
  */
 fun waterfallPlot(
     data: Map<*, *>,

plot-api/src/commonMain/kotlin/org/jetbrains/letsPlot/bistro/waterfall/WaterfallPlotBuilder.kt

@@ -82,7 +82,7 @@ internal class WaterfallPlotBuilder(
                 Waterfall.LABEL_FORMAT to labelFormat,
                 Waterfall.BACKGROUND_LAYERS to when (backgroundLayers) {
                     is Layer -> listOf(backgroundLayers.toSpec())
-                    is List<*> -> backgroundLayers.map {
+                    is FeatureList -> backgroundLayers.elements.map {
                         when (it) {
                             is Layer -> it.toSpec()
                             else -> error("Invalid type in backgroundLayers list")