API: Line2D and scatter default to automatic

 - for plot, the default marker edge is now the color of the marker
 - for scatter the marker edge color now follows the face color

closes matplotlib#4679

Author: tacaswell

lib/matplotlib/axes/_axes.py

@@ -3762,11 +3762,16 @@ def scatter(self, x, y, s=20, c=None, marker='o', cmap=None, norm=None,
             If None, defaults to (lines.linewidth,).
 
         edgecolors : color or sequence of color, optional, default: None
-            If None, defaults to (patch.edgecolor).
+            If None, defaults to 'face'
+
             If 'face', the edge color will always be the same as
-            the face color.  If it is 'none', the patch boundary will not
-            be drawn.  For non-filled markers, the `edgecolors` kwarg
-            is ignored; color is determined by `c`.
+            the face color.
+
+            If it is 'none', the patch boundary will not
+            be drawn.
+
+            For non-filled markers, the `edgecolors` kwarg
+            is ignored and forced to 'face' internally.
 
         Returns
         -------
@@ -3823,6 +3828,9 @@ def scatter(self, x, y, s=20, c=None, marker='o', cmap=None, norm=None,
             else:
                 c = 'b'  # The original default
 
+        if edgecolors is None and not rcParams['_internal.classic_mode']:
+            edgecolors = 'face'
+
         self._process_unit_info(xdata=x, ydata=y, kwargs=kwargs)
         x = self.convert_xunits(x)
         y = self.convert_yunits(y)

lib/matplotlib/lines.py

@@ -849,12 +849,12 @@ def get_marker(self):
     def get_markeredgecolor(self):
         mec = self._markeredgecolor
         if (is_string_like(mec) and mec == 'auto'):
-            if self._marker.get_marker() in ('.', ','):
-                return self._color
-            if self._marker.is_filled() and self.get_fillstyle() != 'none':
-                return 'k'  # Bad hard-wired default...
-            else:
-                return self._color
+            if rcParams['_internal.classic_mode']:
+                if self._marker.get_marker() in ('.', ','):
+                    return self._color
+                if self._marker.is_filled() and self.get_fillstyle() != 'none':
+                     return 'k'  # Bad hard-wired default...
+            return self._color
         else:
             return mec