Fix hue_order as a subset in scatterplot (#2848)

Fixes #2590

doc/releases/v0.12.0.txt

@@ -61,6 +61,8 @@ Other updates
 
 - |Fix| Fixed a bug in :class:`PairGrid` where and error would be raised when defining `hue` only in the mapping methods (:pr:`2847`).
 
+- |Fix| Fixed a bug in :func:`scatterplot` where an error would be raised when `hue_order` was a subset of the hue levels (:pr:`2848`).
+
 - |Fix| Subplot titles will no longer be reset when calling :meth:`FacetGrid.map` or :meth:`FacetGrid.map_dataframe` (:pr:`2705`).
 
 - |Fix| In :func:`lineplot`, allowed the `dashes` keyword to set the style of a line without mapping a `style` variable (:pr:`2449`).

seaborn/_oldcore.py

@@ -149,6 +149,13 @@ def _lookup_single(self, key):
             # Use a value that's in the original data vector
             value = self.lookup_table[key]
         except KeyError:
+
+            if self.norm is None:
+                # Currently we only get here in scatterplot with hue_order,
+                # because scatterplot does not consider hue a grouping variable
+                # So unused hue levels are in the data, but not the lookup table
+                return (0, 0, 0, 0)
+
             # Use the colormap to interpolate between existing datapoints
             # (e.g. in the context of making a continuous legend)
             try:

tests/test_relational.py

@@ -9,6 +9,7 @@
 
 from seaborn.external.version import Version
 from seaborn.palettes import color_palette
+from seaborn._oldcore import categorical_order
 
 from seaborn.relational import (
     _RelationalPlotter,
@@ -1623,6 +1624,16 @@ def test_supplied_color_array(self, long_df):
         _draw_figure(ax.figure)
         assert_array_equal(ax.collections[0].get_facecolors(), colors)
 
+    def test_hue_order(self, long_df):
+
+        order = categorical_order(long_df["a"])
+        unused = order.pop()
+
+        ax = scatterplot(data=long_df, x="x", y="y", hue="a", hue_order=order)
+        points = ax.collections[0]
+        assert (points.get_facecolors()[long_df["a"] == unused] == 0).all()
+        assert [t.get_text() for t in ax.legend_.texts] == order
+
     def test_linewidths(self, long_df):
 
         f, ax = plt.subplots()