Bug summary
The checkboxes in CheckButtons widget are invisible. While the location of the checkboxes are somewhat implied, it is not entirely clear where the clickable region is once CheckButtons is initialized with different axes values.

Code for reproduction
http://matplotlib.org/examples/widgets/check_buttons.html
It can be shown in the output linked above that the checkboxes do not have any outlines.

Expected outcome
I would expect that the checkboxes have a visual outline indicator.

Matplotlib version
MPL 2.0.0 installed from pip, Python 2.7.13, OSX

The suggested fix is simply adding edgecolor='black' to the Rectangle initializer within CheckButtons class:
e.g. in line 542 in widgets.py
p = Rectangle(xy=(x, y), width=w, height=h, edgecolor='black',
facecolor=axcolor,
transform=ax.transAxes)