FIX: Prevent set_alpha from changing color of legend patch

of polycollection (e.g. from fill_between) by updating original
edge and face color too.

Author: StefRe

lib/matplotlib/legend_handler.py

@@ -788,6 +788,8 @@ def get_first(prop_array):
         # Directly set Patch color attributes (must be RGBA tuples).
         legend_handle._facecolor = first_color(orig_handle.get_facecolor())
         legend_handle._edgecolor = first_color(orig_handle.get_edgecolor())
+        legend_handle._original_facecolor = orig_handle._original_facecolor
+        legend_handle._original_edgecolor = orig_handle._original_edgecolor
         legend_handle._fill = orig_handle.get_fill()
         legend_handle._hatch = orig_handle.get_hatch()
         # Hatch color is anomalous in having no getters and setters.

lib/matplotlib/tests/test_legend.py

@@ -879,3 +879,11 @@ def test_subfigure_legend():
     ax.plot([0, 1], [0, 1], label="line")
     leg = subfig.legend()
     assert leg.figure is subfig
+
+
+def test_setting_alpha_keeps_polycollection_color():
+    pc = plt.fill_between([0, 1], [2, 3], color='#123456', label='label')
+    patch = plt.legend().get_patches()[0]
+    patch.set_alpha(0.5)
+    assert patch.get_facecolor()[:3] == tuple(pc.get_facecolor()[0][:3])
+    assert patch.get_edgecolor()[:3] == tuple(pc.get_edgecolor()[0][:3])