Matplotlib标准化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

add this after plt.colorbar() :

plt.clim(minimal_value, maximal_value)

for the contour plot, add the args vmin and vmax:

cont_PSD = plt.contourf(plotFreq, plotCoord, plotPxx, 200, linestyle=None,vmin=minimal_value,vmax=maximal_value)

You complete code should work like this :

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,vmin=minimal_value,vmax=maximal_value);


plt.colorbar()
plt.clim(minimal_value,maximal_value)
plt.show()

Answer 2

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!

Answer 3

Please user the levels parameter, a set of examples:

In [9]:

ndom
z = np.random.random((10,10))

Without levels , colorbar will be auto-scaled

In [11]:
plt.contourf(z)
plt.colorbar()
Out[11]:
<matplotlib.colorbar.Colorbar at 0x120d47390>

In [12]:
plt.contourf(z*2)
plt.colorbar()
Out[12]:
<matplotlib.colorbar.Colorbar at 0x120f6ac10>

Control colorbar with explicit levels

In [13]:
plt.contourf(z*2, levels=np.linspace(0,2,20))
plt.colorbar()
Out[13]:
<matplotlib.colorbar.Colorbar at 0x121b119d0>

In [14]:
plt.contourf(z, levels=np.linspace(0,2,20))
plt.colorbar()
Out[14]:
<matplotlib.colorbar.Colorbar at 0x120dc3510>