fix(geom_rect): draw each rectangle as its own polygon in non-linear coords

In a non-linear coord, rectangles are drawn as polygons grouped by the
`group` aesthetic. A stacked bar shares one group across its segments, so
they merged into a single open path and coord.munch bent the join between
consecutive rectangles into a triangular spike. Regroup into one polygon
per rectangle so each is closed independently.

Author: has2k1

doc/changelog.qmd

@@ -112,6 +112,11 @@ title: Changelog
 
 ### Bug Fixes
 
+- Stacked bars in a non-linear coordinate system (e.g.
+  [](:class:`~plotnine.coord_trans`)) no longer show triangular spikes; each
+  rectangle is now drawn as its own polygon instead of merging a bar's
+  segments into a single path.
+
 - 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_rect.py

@@ -54,6 +54,11 @@ def draw_panel(
         """
         if not coord.is_linear:
             data = _rectangles_to_polygons(data)
+            # Each rectangle is its own closed polygon. Grouping by the
+            # `group` aesthetic would merge a stacked bar's segments into one
+            # open path, and coord.munch would then bend the join between
+            # consecutive rectangles into a spurious spike.
+            data["group"] = np.repeat(np.arange(len(data) // 4), 4)
             for _, gdata in data.groupby("group"):
                 gdata.reset_index(inplace=True, drop=True)
                 geom_polygon.draw_group(

tests/baseline_images/test_coords/coord_trans_stacked_bars_have_no_spikes.png

@@ -12,11 +12,13 @@
     coord_flip,
     coord_trans,
     geom_bar,
+    geom_col,
     geom_line,
     geom_point,
     ggplot,
     xlim,
 )
+from plotnine.data import mtcars
 
 n = 10  # Some even number greater than 2
 
@@ -74,6 +76,14 @@ def test_coord_trans_backtransforms():
     assert p == "coord_trans_backtransform"
 
 
+def test_coord_trans_stacked_bars_have_no_spikes():
+    # Each stacked segment must be its own polygon. If they merge into one
+    # path, the join between consecutive segments becomes a diagonal that
+    # munch subdivides into a triangular spike across the bar.
+    p = ggplot(mtcars, aes("factor(cyl)", "mpg")) + geom_col() + coord_trans()
+    assert p == "coord_trans_stacked_bars_have_no_spikes"
+
+
 def test_datetime_coord_limits():
     n = 6