ENH: violins to overlap top to bottom from the origin

Author: has2k1

doc/changelog.qmd

@@ -80,6 +80,10 @@ title: Changelog
   [](:attr:`~plotnine.data.midwest`) no longer have any of their columns as categoricals.
   This matches the respective datasets in R. {{< issue 913 >}}
 
+- When the `width` of the violins in [](:class:`~plotnine.geom_violin`) is wider than their
+  normal area, they now overlap with those closest to the origin on top. This makes it possible
+  to create overlapping ridge plots using half violins.
+
 ### Bug Fixes
 
 - Fixed bug in with the `legend_key_height` themeable where it wasn't applied.

plotnine/geoms/geom_violin.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-import typing
+from typing import TYPE_CHECKING, cast
 
 import numpy as np
 import pandas as pd
@@ -11,7 +11,7 @@
 from .geom_path import geom_path
 from .geom_polygon import geom_polygon
 
-if typing.TYPE_CHECKING:
+if TYPE_CHECKING:
     from typing import Any
 
     from matplotlib.axes import Axes
@@ -115,10 +115,17 @@ def draw_panel(
         ax: Axes,
         **params: Any,
     ):
-        quantiles = params["draw_quantiles"]
-        style = params["style"]
+        quantiles = params.pop("draw_quantiles")
+        style = params.pop("style")
+        zorder = params.pop("zorder")
+
+        for i, (group, df) in enumerate(data.groupby("group")):
+            # Place the violins with the smalleer group number on top
+            # of those with larger numbers. The group_zorder values should be
+            # in the range [zorder, zorder + 1) to stay within the layer.
+            group = cast("int", group)
+            group_zorder = zorder + 0.9 / group
 
-        for i, (_, df) in enumerate(data.groupby("group")):
             # Find the points for the line to go all the way around
             df["xminv"] = df["x"] - df["violinwidth"] * (df["x"] - df["xmin"])
             df["xmaxv"] = df["x"] + df["violinwidth"] * (df["xmax"] - df["x"])
@@ -156,7 +163,12 @@ def draw_panel(
 
             # plot violin polygon
             geom_polygon.draw_group(
-                polygon_df, panel_params, coord, ax, **params
+                polygon_df,
+                panel_params,
+                coord,
+                ax,
+                zorder=group_zorder,
+                **params,
             )
 
             if quantiles is not None:
@@ -174,7 +186,12 @@ def draw_panel(
 
                 # plot quantile segments
                 geom_path.draw_group(
-                    segment_df, panel_params, coord, ax, **params
+                    segment_df,
+                    panel_params,
+                    coord,
+                    ax,
+                    zorder=group_zorder,
+                    **params,
                 )
 
 

tests/baseline_images/test_geom_violin/overlap.png

@@ -132,3 +132,11 @@ def test_scales_free():
         + facet_grid("am", "'column'", scales="free")
     )
     assert p == "scales_free"
+
+
+def test_overlap():
+    p = ggplot(data, aes("x")) + geom_violin(
+        aes(y="y", fill="x"), position="identity", width=2
+    )
+
+    assert p == "overlap"