Make default facecolor for subfigures be transparent ("none"). Fix for issue matplotlib#24910 (matplotlib#25255)

* RR - Set default facecolor for subfigures to be none (Transparent). Fixes issue matplotlib#24910

* RR - Resolve merge conflict

Author: robrighter

doc/api/next_api_changes/behavior/25255-RR.rst

@@ -0,0 +1,5 @@
+``SubFigure`` default facecolor is now transparent
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Subfigures default facecolor changed to ``"none"``. Previously the default was
+the value of ``figure.facecolor``.

lib/matplotlib/figure.py

@@ -2155,8 +2155,8 @@ def __init__(self, parent, subplotspec, *,
             Defines the region in a parent gridspec where the subfigure will
             be placed.
 
-        facecolor : default: :rc:`figure.facecolor`
-            The figure patch face color.
+        facecolor : default: ``"none"``
+            The figure patch face color; transparent by default.
 
         edgecolor : default: :rc:`figure.edgecolor`
             The figure patch edge color.
@@ -2176,7 +2176,7 @@ def __init__(self, parent, subplotspec, *,
         """
         super().__init__(**kwargs)
         if facecolor is None:
-            facecolor = mpl.rcParams['figure.facecolor']
+            facecolor = "none"
         if edgecolor is None:
             edgecolor = mpl.rcParams['figure.edgecolor']
         if frameon is None:

lib/matplotlib/tests/test_figure.py

@@ -287,6 +287,20 @@ def test_suptitle_fontproperties():
     assert txt.get_weight() == fps.get_weight()
 
 
+def test_suptitle_subfigures():
+    fig = plt.figure(figsize=(4, 3))
+    sf1, sf2 = fig.subfigures(1, 2)
+    sf2.set_facecolor('white')
+    sf1.subplots()
+    sf2.subplots()
+    fig.suptitle("This is a visible suptitle.")
+
+    # verify the first subfigure facecolor is the default transparent
+    assert sf1.get_facecolor() == (0.0, 0.0, 0.0, 0.0)
+    # verify the second subfigure facecolor is white
+    assert sf2.get_facecolor() == (1.0, 1.0, 1.0, 1.0)
+
+
 @image_comparison(['alpha_background'],
                   # only test png and svg. The PDF output appears correct,
                   # but Ghostscript does not preserve the background color.
@@ -1211,12 +1225,14 @@ def test_subfigure():
         pc = ax.pcolormesh(np.random.randn(30, 30), vmin=-2, vmax=2)
     sub[0].colorbar(pc, ax=axs)
     sub[0].suptitle('Left Side')
+    sub[0].set_facecolor('white')
 
     axs = sub[1].subplots(1, 3)
     for ax in axs.flat:
         pc = ax.pcolormesh(np.random.randn(30, 30), vmin=-2, vmax=2)
     sub[1].colorbar(pc, ax=axs, location='bottom')
     sub[1].suptitle('Right Side')
+    sub[1].set_facecolor('white')
 
     fig.suptitle('Figure suptitle', fontsize='xx-large')