[Matplotlib-users] Bug in pcolor's use of vmax, vmin?

pcolor doesn't seem to be using the vmin and vmax values that I supply
to it.  If I do:

X,Y = meshgrid(arange(-2,2,.1), arange(-2,2,.1))
pcolor(X**2 + Y**2, vmin = 1., vmax=2.)

I see a smooth paraboloid (ie, without the min/max clipping in the
center on the edge that I would expect).

However, if I instead do:

imshow(X**2 + Y**2, vmin = 1., vmax=2.)

I see what I expect, a circular dark region, surrounded by a smoothly
varying ring, surrounded by a white area to the edge of the plot.  

I'm using OS X 10.4.5, Python 2.3.5, and matplotlib 0.87.  Am I doing
something wrong?

Thanks,
Greg