Avoid error when dodge=True, hue=None (#3605)

Fixes #3553

seaborn/categorical.py

@@ -392,6 +392,11 @@ def _dodge_needed(self):
 
     def _dodge(self, keys, data):
         """Apply a dodge transform to coordinates in place."""
+        if "hue" not in self.variables:
+            # Short-circuit if hue variable was not assigned
+            # We could potentially warn when hue=None, dodge=True, user may be confused
+            # But I think it's fine to just treat it as a no-op.
+            return
         hue_idx = self._hue_map.levels.index(keys["hue"])
         n = len(self._hue_map.levels)
         data["width"] /= n

tests/test_categorical.py

@@ -999,6 +999,16 @@ def test_dodge_native_scale_log(self, long_df):
                 widths.append(np.ptp(coords))
         assert np.std(widths) == approx(0)
 
+    def test_dodge_without_hue(self, long_df):
+
+        ax = boxplot(long_df, x="a", y="y", dodge=True)
+        bxp, = ax.containers
+        levels = categorical_order(long_df["a"])
+        for i, level in enumerate(levels):
+            data = long_df.loc[long_df["a"] == level, "y"]
+            self.check_box(bxp[i], data, "x", i)
+            self.check_whiskers(bxp[i], data, "x", i)
+
     @pytest.mark.parametrize("orient", ["x", "y"])
     def test_log_data_scale(self, long_df, orient):