FIX: Keep legacy alpha behavior for violinplot without facecolor

Closes matplotlib#30613.

Author: timhoffm

lib/matplotlib/axes/_axes.py

@@ -8967,6 +8967,14 @@ def violin(self, vpstats, positions=None, vert=None,
 
             .. versionadded:: 3.11
 
+            For backward compatibility, if *facecolor* is not given, the body
+            will get an Artist-level transparency `alpha <.Artist.set_alpha>`
+            of 0.3, which will persist if you afterwards change the facecolor,
+            e.g. via ``result['bodies'][0].set_facecolor('red')``.
+            If *facecolor* is given, there is no Artist-level transparency.
+            You can use the ``(color, alpha)`` tuple notation to pass
+            semi-transparent colors.
+
         linecolor : :mpltype:`color` or list of :mpltype:`color`, optional
             If provided, will set the line color(s) of the violins (the
             horizontal and vertical spines and body edges).
@@ -9074,13 +9082,14 @@ def cycle_color(color, alpha=None):
 
         if facecolor is not None:
             facecolor = cycle_color(facecolor)
+            body_artist_alpha = None
         else:
-            default_facealpha = 0.3
+            body_artist_alpha = 0.3
             # Use default colors if user doesn't provide them
             if mpl.rcParams['_internal.classic_mode']:
-                facecolor = cycle_color('y', alpha=default_facealpha)
+                facecolor = cycle_color('y')
             else:
-                facecolor = cycle_color(next_color, alpha=default_facealpha)
+                facecolor = cycle_color(next_color)
 
         if mpl.rcParams['_internal.classic_mode']:
             # Classic mode uses patch.force_edgecolor=True, so we need to
@@ -9129,7 +9138,8 @@ def cycle_color(color, alpha=None):
             bodies += [fill(stats['coords'],
                             -vals + pos if side in ['both', 'low'] else pos,
                             vals + pos if side in ['both', 'high'] else pos,
-                            facecolor=facecolor, edgecolor=body_edgecolor)]
+                            facecolor=facecolor, edgecolor=body_edgecolor,
+                            alpha=body_artist_alpha)]
             means.append(stats['mean'])
             mins.append(stats['min'])
             maxes.append(stats['max'])

lib/matplotlib/tests/test_axes.py

@@ -4228,6 +4228,33 @@ def assert_colors_equal(colors1, colors2):
         assert_colors_equal(colors_test, mcolors.to_rgba_array(linecolors))
 
 
+def test_violinplot_alpha():
+    matplotlib.style.use('default')
+    data = [(np.random.normal(0, 1, 100))]
+
+    fig, ax = plt.subplots()
+    parts = ax.violinplot(data, positions=[1])
+
+    # Case 1: If facecolor is unspecified, it's the first color from the color cycle
+    # with Artist-level alpha=0.3
+    facecolor = ('y' if mpl.rcParams['_internal.classic_mode']
+                 else plt.rcParams['axes.prop_cycle'].by_key()['color'][0])
+    assert mcolors.same_color(parts['bodies'][0].get_facecolor(), (facecolor, 0.3))
+    assert parts['bodies'][0].get_alpha() == 0.3
+    # setting a new facecolor maintains the alpha
+    parts['bodies'][0].set_facecolor('red')
+    assert mcolors.same_color(parts['bodies'][0].get_facecolor(), ('red', 0.3))
+
+    # Case 2: If facecolor is explicitly given, it's alpha does not become an
+    # Artist property
+    parts = ax.violinplot(data, positions=[1], facecolor=('blue', 0.3))
+    assert mcolors.same_color(parts['bodies'][0].get_facecolor(), ('blue', 0.3))
+    assert parts['bodies'][0].get_alpha() is None
+    # so setting a new color does not maintain the alpha
+    parts['bodies'][0].set_facecolor('red')
+    assert mcolors.same_color(parts['bodies'][0].get_facecolor(), 'red')
+
+
 @check_figures_equal()
 def test_violinplot_single_list_quantiles(fig_test, fig_ref):
     # Ensures quantile list for 1D can be passed in as single list