BUG : correct default value alpha on legend patch

Fixes matplotlib#3766

Adding `legend.framealpha` in matplotlib#3369 changed the default value of
alpha on the legend patch from `None` -> 1 which has no visual effect
as `to_rgba` defaults to alpha=1 if no alpha is passed, but this
overrides setting the alpha component via `set_facecolor(rgba_value)`.

This PR

 - addes a new validation function `validate_float_or_None`
 - changes legend.framealpha to default to `None`
 - adds test that with the default value of `legend.framealpha`
   the alpha value of the face can be set via a rgba value
 - adds tests that setting the framealpha to a non-None value
   ignores the alpha value passed to set_facecolor

Author: tacaswell

lib/matplotlib/rcsetup.py

@@ -108,6 +108,16 @@ def validate_float(s):
         raise ValueError('Could not convert "%s" to float' % s)
 
 
+def validate_float_or_None(s):
+    """convert s to float or raise"""
+    if s is None:
+        return None
+    try:
+        return float(s)
+    except ValueError:
+        raise ValueError('Could not convert "%s" to float' % s)
+
+
 def validate_int(s):
     """convert s to int or raise"""
     try:
@@ -640,7 +650,7 @@ def __call__(self, s):
      # whether or not to draw a frame around legend
     'legend.frameon': [True, validate_bool],
      # alpha value of the legend frame
-    'legend.framealpha': [1.0, validate_float],
+    'legend.framealpha': [None, validate_float_or_None],
 
     ## the following dimensions are in fraction of the font size
     'legend.borderpad': [0.4, validate_float],  # units are fontsize

lib/matplotlib/tests/baseline_images/test_legend/rcparam_alpha.png

@@ -64,6 +64,33 @@ def test_various_labels():
     ax.legend(numpoints=1, loc=0)
 
 
+@image_comparison(baseline_images=['rgba_alpha'],
+                  extensions=['png'], remove_text=True)
+def test_alpha_rgba():
+    import matplotlib.pyplot as plt
+
+    fig, ax = plt.subplots(1, 1)
+    ax.plot(range(10), lw=5)
+    leg = plt.legend(['Longlabel that will go away'], loc=10)
+    leg.legendPatch.set_facecolor([1, 0, 0, 0.5])
+
+
+@image_comparison(baseline_images=['rcparam_alpha'],
+                  extensions=['png'], remove_text=True)
+def test_alpha_rcparam():
+    import matplotlib.pyplot as plt
+
+    fig, ax = plt.subplots(1, 1)
+    ax.plot(range(10), lw=5)
+    with mpl.rc_context(rc={'legend.framealpha': .75}):
+        leg = plt.legend(['Longlabel that will go away'], loc=10)
+        # this alpha is going to be over-ridden by the rcparam whith
+        # sets the alpha of the patch to be non-None which causes the alpha
+        # value of the face color to be discarded.  This behavior may not be
+        # ideal, but it is what it is and we should keep track of it changing
+        leg.legendPatch.set_facecolor([1, 0, 0, 0.5])
+
+
 @image_comparison(baseline_images=['fancy'], remove_text=True)
 def test_fancy():
     # using subplot triggers some offsetbox functionality untested elsewhere