fix(plotly): Dict as cmap for categorical data (#6940)

Author: hoxbro

holoviews/plotting/plotly/element.py

@@ -440,6 +440,17 @@ def _apply_transforms(self, element, ranges, style):
                     if not is_plotly_allowed_colors:
                         val = cat_indices
 
+                        cmap = style.get("cmap")
+                        if isinstance(cmap, dict):
+                            style = dict(style)
+                            style["cmap"] = [c for cat in categories if (c := cmap.get(cat))]
+                            if len(style["cmap"]) != len(categories):
+                                missing = ", ".join(
+                                    map(repr, sorted(set(categories) - set(cmap.keys())))
+                                )
+                                msg = f"Missing color(s) for categories: {missing}"
+                                raise ValueError(msg)
+
                         copts = self.get_color_opts(v, element, ranges, style)
                         copts.update({"cmin": 0, "cmax": len(categories) - 1, "cauto": False})
 

holoviews/tests/plotting/plotly/test_scatterplot.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import numpy as np
+import pytest
 
 import holoviews as hv
 from holoviews.plotting.plotly.util import PLOTLY_MAP, PLOTLY_SCATTERMAP
@@ -62,6 +63,24 @@ def test_scatter_categorical_color(self):
         assert state["data"][0]["marker"]["cmin"] == 0
         assert state["data"][0]["marker"]["cmax"] == 2
 
+    def test_scatter_categorical_color_cmap_dict(self):
+        scatter = hv.Scatter([(0, 1, "A"), (1, 2, "B")], vdims=["y", "category"])
+        scatter.opts(color="category", cmap={"A": "black", "B": "red"})
+        state = self._get_plot_state(scatter)
+
+        marker = state["data"][0]["marker"]
+        np.testing.assert_array_equal(marker["color"], [0, 1])
+        assert marker["colorscale"] == [[0, "black"], [1, "red"]]
+        assert marker["cmin"] == 0
+        assert marker["cmax"] == 1
+
+    def test_scatter_categorical_color_cmap_dict_missing(self):
+        scatter = hv.Scatter([(0, 1, "A"), (1, 2, "B")], vdims=["y", "category"])
+        scatter.opts(color="category", cmap={"A": "black"})
+        msg = "Missing color\\(s\\) for categories: 'B'"
+        with pytest.raises(ValueError, match=msg):
+            self._get_plot_state(scatter)
+
     def test_scatter_markers(self):
         scatter = hv.Scatter(
             [(0, 1, "square"), (1, 2, "circle"), (2, 3, "triangle-up")], vdims=["y", "marker"]