title: Defining the midpoint of a colormap in matplotlib
body: <p>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?</p>