[Matplotlib-users] plotting a pcolormesh and fill

I have a pcolormest and a fill that are plotted on the same graph (they hav=
e
the same scale).  Is there an easy way
to only plot the values of pcolormesh that lie in the fill?

pcolormesh(oX, oY, test, shading=3D'flat', cmap=3Dcm.gray_r)
fill([-x1,x2,x2,-x1], [-y1,-y1,y2,y2], 'b', alpha=3D0.2, edgecolor=3D'r')


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"The game of science can accurately be described as a never-ending insult t=
o
human intelligence." - Jo=E3o Magueijo