fix(geom_polygon): munch the closing edge in non-linear coords

doc/changelog.qmd

@@ -112,6 +112,10 @@ title: Changelog
 
 ### Bug Fixes
 
+- In a non-linear coordinate system (e.g. [](:class:`~plotnine.coord_trans`)),
+  the closing edge of a polygon is now curved along with the rest of its
+  boundary instead of being drawn as a straight chord.
+
 - The space between facet panels now accounts for the margins of the axis
   text, so with free scales large margins no longer push the tick labels
   into the neighbouring panel.

plotnine/geoms/geom_polygon.py

@@ -73,6 +73,19 @@ def draw_group(
     ):
         from matplotlib.collections import PolyCollection
 
+        # A polygon is a closed ring, but the vertices only trace it open.
+        # In a non-linear coord the closing edge must be munched like any
+        # other, or it stays a straight chord across a curved boundary (e.g.
+        # the inner arc of a polar bar). Repeat each group's first vertex so
+        # munch subdivides that edge too; a linear coord closes it straight,
+        # which is already correct, so leave it untouched there.
+        if not coord.is_linear:
+            indices = data.groupby("group", sort=False).indices
+            order = np.concatenate(
+                [np.append(idx, idx[0]) for idx in indices.values()]
+            )
+            data = data.iloc[order].reset_index(drop=True)
+
         data = coord.transform(data, panel_params, munch=True)
         data["linewidth"] = data["size"] * SIZE_FACTOR
 

tests/test_coords.py

@@ -14,6 +14,7 @@
     geom_bar,
     geom_line,
     geom_point,
+    geom_polygon,
     ggplot,
     xlim,
 )
@@ -74,6 +75,21 @@ def test_coord_trans_backtransforms():
     assert p == "coord_trans_backtransform"
 
 
+def test_coord_trans_munches_polygon_closing_edge():
+    # A polygon is a closed ring, but its vertices only trace it open; the
+    # edge from the last vertex back to the first must be munched like any
+    # other. The triangle's two explicit edges are axis-aligned (straight
+    # under log), so only the closing edge curves — if it is left as a
+    # straight chord this baseline shows a triangle instead of the arc.
+    tri = pd.DataFrame({"x": [1.0, 1.0, 100.0], "y": [100.0, 1.0, 1.0]})
+    p = (
+        ggplot(tri, aes("x", "y"))
+        + geom_polygon(fill="none", color="black", size=1)
+        + coord_trans(x="log10", y="log10")
+    )
+    assert p == "coord_trans_munches_polygon_closing_edge"
+
+
 def test_datetime_coord_limits():
     n = 6