Merge pull request matplotlib#413 from leejjoon/set_marker_none

restore 'None' (a string) as a valid marker style (draw nothing)

Author: mdboom

lib/matplotlib/markers.py

@@ -90,6 +90,7 @@ class MarkerStyle:
         CARETRIGHT : 'caretright',
         CARETUP    : 'caretup',
         CARETDOWN  : 'caretdown',
+        "None"       : 'nothing',
         None       : 'nothing',
         ' '        : 'nothing',
         ''         : 'nothing'