Matplotlib normalize colorbar (Python)

Question

I'm trying to plot a contourf-plot using matplotlib (and numpy of course). And it works, it plots what it should plot, but unfortunatelly I cannot set the colorbar range. The problem is that I have a plenty of plots and need all of them to have the same colorbar (same min and max, same colors). I copy&past-ed almost every code snippet I found on the internet, but without success. My code so far:

    import numpy as np;
    import matplotlib as mpl;
    import matplotlib.pyplot as plt;
    [...]
    plotFreq, plotCoord = np.meshgrid(plotFreqVect, plotCoordVect);

    figHandler = plt.figure();
    cont_PSD = plt.contourf(plotFreq, plotCoord, plotPxx, 200, linestyle=None);


    normi = mpl.colors.Normalize(vmin=-80, vmax=20);

    colbar_PSD = plt.colorbar(cont_PSD);
    colbar_PSD.set_norm(normi);
    #colbar_PSD.norm = normi;
    #mpl.colors.Normalize(vmin=-80, vmax=20);

    plt.axis([1, 1000, -400, 400]);

As you can see there are three different lines for the colorbar norm, none of them is working. The range is still set automatically... I mean everything else is working, why not the colorbar? I don't even get errors or warnings.

Thanks, itpdg

EDIT 1: Pictures, with plt.clim(-80,20):

Answer 1

I ran into this issue a while back and thought it was a bug (see MPL issue #5055). It's not, but it does require using the extend kwarg, which was non-intuitive to me. Here's what you want to do:

normi = mpl.colors.Normalize(vmin=-80, vmax=20)

cont_PSD = plt.contourf(plotFreq, plotCoord, plotPxx,
                        np.linspace(-80, 20, 200),
                        linestyle=None,
                        norm=normi, extend='both')

plt.colorbar(colbar_PSD)

You can do-away with the plt.clim, colbar_PSD.set_norm and other similar calls.

More examples uses of extend= are available here.

Note that this will create a colorbar with 'triangles' at the top and bottom indicating that the data extends beyond the colorbar, but I think you'll like them once you get used to them, they are descriptive.

Good luck!