'Override this method to define the shape that is used for the
background of the plot.  It should be a subclass of Patch.
In this case, it is a Circle (that may be warped by the axes
transform into an ellipse).  Any data and gridlines will be
clipped to this shape.'

def _gen_axes_patch(self):

    return Circle((0.5, 0.5), 0.5)
