colormap

I want to set the middle point of a colormap, i.e., my data goes from -5 to 10 and I want zero to be the middle point. I think the way to do it is by subclassing normalize and using the norm, but I didn’t find any example and it is not clear to me, what exactly have I to implement?

I’d like to make an overlay of several hexbin plots, but with builtin colormaps only the last one is visible. I don’t want to construct a colormap de novo. How one would add linear alpha to the colormap without knowing the inner structure of the colormap beforehand?