Fix Plot legend with > 2 layers

Fixes #3023

This ended up being a simple typo :(

Author: mwaskom

seaborn/_core/plot.py

@@ -1582,7 +1582,7 @@ def _make_legend(self, p: Plot) -> None:
         merged_contents: dict[
             tuple[str, str | int], tuple[list[Artist], list[str]],
         ] = {}
-        for key, artists, labels in self._legend_contents:
+        for key, new_artists, labels in self._legend_contents:
             # Key is (name, id); we need the id to resolve variable uniqueness,
             # but will need the name in the next step to title the legend
             if key in merged_contents:
@@ -1591,11 +1591,11 @@ def _make_legend(self, p: Plot) -> None:
                 for i, artist in enumerate(existing_artists):
                     # Matplotlib accepts a tuple of artists and will overlay them
                     if isinstance(artist, tuple):
-                        artist += artist[i],
+                        artist += new_artists[i],
                     else:
-                        existing_artists[i] = artist, artists[i]
+                        existing_artists[i] = artist, new_artists[i]
             else:
-                merged_contents[key] = artists.copy(), labels
+                merged_contents[key] = new_artists.copy(), labels
 
         # TODO explain
         loc = "center right" if self._pyplot else "center left"

tests/_core/test_plot.py

@@ -1963,6 +1963,20 @@ def _legend_artist(self, variables, value, scales):
             assert len(contents.findobj(mpl.lines.Line2D)) == len(names)
             assert len(contents.findobj(mpl.patches.Patch)) == len(names)
 
+    def test_three_layers(self, xy):
+
+        class MockMarkLine(MockMark):
+            def _legend_artist(self, variables, value, scales):
+                return mpl.lines.Line2D([], [])
+
+        s = pd.Series(["a", "b", "a", "c"], name="s")
+        p = Plot(**xy, color=s)
+        for _ in range(3):
+            p = p.add(MockMarkLine())
+        p = p.plot()
+        texts = p._figure.legends[0].get_texts()
+        assert len(texts) == len(s.unique())
+
     def test_identity_scale_ignored(self, xy):
 
         s = pd.Series(["r", "g", "b", "g"])