From 2ce6fdb53f953bf966e1aa2155f0efe425381516 Mon Sep 17 00:00:00 2001
From: Hassan Kibirige <has2k1@gmail.com>
Date: Wed, 15 Jul 2026 19:20:32 +0300
Subject: [PATCH] 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.
---
 doc/changelog.qmd                                |   5 +++++
 plotnine/geoms/geom_rect.py                      |   5 +++++
 .../coord_trans_stacked_bars_have_no_spikes.png  | Bin 0 -> 2903 bytes
 tests/test_coords.py                             |  10 ++++++++++
 4 files changed, 20 insertions(+)
 create mode 100644 tests/baseline_images/test_coords/coord_trans_stacked_bars_have_no_spikes.png

diff --git a/doc/changelog.qmd b/doc/changelog.qmd
index 46f687e24..0afaea656 100644
--- a/doc/changelog.qmd
+++ b/doc/changelog.qmd
@@ -116,6 +116,11 @@ title: Changelog
   the closing edge of a polygon is now curved along with the rest of its
   boundary instead of being drawn as a straight chord.
 
+- 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.
diff --git a/plotnine/geoms/geom_rect.py b/plotnine/geoms/geom_rect.py
index eecae786c..9b52ecf8e 100644
--- a/plotnine/geoms/geom_rect.py
+++ b/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(
diff --git a/tests/baseline_images/test_coords/coord_trans_stacked_bars_have_no_spikes.png b/tests/baseline_images/test_coords/coord_trans_stacked_bars_have_no_spikes.png
new file mode 100644
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diff --git a/tests/test_coords.py b/tests/test_coords.py
index 0525e9d88..4d2e1fde6 100644
--- a/tests/test_coords.py
+++ b/tests/test_coords.py
@@ -12,12 +12,14 @@
     coord_flip,
     coord_trans,
     geom_bar,
+    geom_col,
     geom_line,
     geom_point,
     geom_polygon,
     ggplot,
     xlim,
 )
+from plotnine.data import mtcars
 
 n = 10  # Some even number greater than 2
 
@@ -90,6 +92,14 @@ def test_coord_trans_munches_polygon_closing_edge():
     assert p == "coord_trans_munches_polygon_closing_edge"
 
 
+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