Line2D markfacecolor=None fails if set_alpha() is called #11113

Bug report

Calling Line2D with markerfacecolor='None' should yield a marker with just an outline. That works fine. But if a call to artist.set_alpha( X ) where is X is anything but None is made, then the marker is filled no matter what with black color and alpha=X.

Code for reproduction

import pylab as p
p.ion()
a=p.plot( [1,2], marker='o' )[0]

# Works, marker is unfilled
a.set_markerfacecolor( 'None' )

# Fails - marker is filled with grey
a.set_alpha( 0.5 )

Actual outcome

Matplotlib version

• Operating system: RedHat Linux 7
• Matplotlib version: 2.2.2
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: 2.7