Thread Subject: mapshow + pcolor?

I have successfully plotted a geotiff using mapshow. I would next like to overlay gridded data using pcolor, but it doesn't work and doesn't provide an error message. Instead, the plot becomes blank. Are these commands fundamentally incompatible? contourf and its ilk can be overlaid, but filled contours cannot be displayed transparently, so I cannot see the underlying geotiff? Any ideas? An example of the code:

[A, A_colormap, A_ref, A_box] = geotiffread('A.tif');
figure;
mapshow(A, A_ref, 'displaytype', 'image'); % doesn't really need the displaytype switch
axis equal;
hold on;
pcolor(x, y, z);

Thanks,

Joe