Make set_marker{edge,face}color(None) more consistent.

Resolving None to mean "the rcParam" is both consistent with the
behavior in `__init__` (which manually maps None to the rcParam) and
with markeredgewidth.  In particular, being consistent with `__init__`
ensures that in
```
from pylab import *; from matplotlib.lines import Line2D
rcParams["lines.markerfacecolor"] = "red"
gca().add_artist(Line2D([.1, .9], [.1, .9], marker="o", markerfacecolor=None))
gca().add_artist(Line2D([.1, .9], [.9, .1], marker="o", mfc=None))
```
both lines use "red" as markerfacecolor (previously, the second one
would go through the setter and use "auto" (i.e. the line's color)
instead).

I guess this should have been done when the rcParams were added in
1d9880c, but missed back then.

(No change for markerfacecoloralt because there's no corresponding
rcParam; perhaps one should be added just for completeness?)

Author: anntzer

doc/api/next_api_changes/behavior/20199-AL.rst

@@ -0,0 +1,5 @@
+``Line2D.set_markeredgecolor(None)`` and ``Line2D.set_markerfacecolor(None)``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... now set the line property using the corresponding rcParam
+(:rc:`lines.markeredgecolor` and :rc:`lines.markerfacecolor`).  This is
+consistent with other `.Line2D` property setters.

lib/matplotlib/lines.py

@@ -321,10 +321,6 @@ def __init__(self, xdata, ydata,
             linestyle = rcParams['lines.linestyle']
         if marker is None:
             marker = rcParams['lines.marker']
-        if markerfacecolor is None:
-            markerfacecolor = rcParams['lines.markerfacecolor']
-        if markeredgecolor is None:
-            markeredgecolor = rcParams['lines.markeredgecolor']
         if color is None:
             color = rcParams['lines.color']
 
@@ -386,9 +382,9 @@ def __init__(self, xdata, ydata,
         self._markerfacecolor = None
         self._markerfacecoloralt = None
 
-        self.set_markerfacecolor(markerfacecolor)
+        self.set_markerfacecolor(markerfacecolor)  # Normalizes None to rc.
         self.set_markerfacecoloralt(markerfacecoloralt)
-        self.set_markeredgecolor(markeredgecolor)
+        self.set_markeredgecolor(markeredgecolor)  # Normalizes None to rc.
         self.set_markeredgewidth(markeredgewidth)
 
         # update kwargs before updating data to give the caller a
@@ -1146,9 +1142,10 @@ def set_marker(self, marker):
         self._marker = MarkerStyle(marker, self._marker.get_fillstyle())
         self.stale = True
 
-    def _set_markercolor(self, attr, val):
+    def _set_markercolor(self, name, has_rcdefault, val):
         if val is None:
-            val = 'auto'
+            val = rcParams[f"lines.{name}"] if has_rcdefault else "auto"
+        attr = f"_{name}"
         current = getattr(self, attr)
         if current is None:
             self.stale = True
@@ -1167,7 +1164,7 @@ def set_markeredgecolor(self, ec):
         ----------
         ec : color
         """
-        self._set_markercolor("_markeredgecolor", ec)
+        self._set_markercolor("markeredgecolor", True, ec)
 
     def set_markerfacecolor(self, fc):
         """
@@ -1177,7 +1174,7 @@ def set_markerfacecolor(self, fc):
         ----------
         fc : color
         """
-        self._set_markercolor("_markerfacecolor", fc)
+        self._set_markercolor("markerfacecolor", True, fc)
 
     def set_markerfacecoloralt(self, fc):
         """
@@ -1187,7 +1184,7 @@ def set_markerfacecoloralt(self, fc):
         ----------
         fc : color
         """
-        self._set_markercolor("_markerfacecoloralt", fc)
+        self._set_markercolor("markerfacecoloralt", False, fc)
 
     def set_markeredgewidth(self, ew):
         """