title: Python: Shifted logarithmic colorbar, white color offset to center
body: <p>I am struggling to plot my data using <code>pyplot.imshow</code>. I use the 'RdBu_r' colormap, and I need the white color to be at value 1 on a logarithmic scale which is not centered at 1. But how can I do it?
  I tried 'center=1' which <a href="https://stackoverflow.com/questions/48589567/python-how-to-centre-a-colorbar-at-a-defined-value-for-seaborn-heatmap">works for seaborn</a>, but there is no such attribute in matplotlib. I also tried this:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  im=plt.imshow(proportion, cmap="RdBu_r", norm=LogNorm(), vmin=0.01, vmax=10)
  axs=plt.gca()
  cb = plt.colorbar(im, ax=axs,extend="both")
  </code></pre>
  
  <p>where <code>proportion</code> is my data array, ranging from 0.01 to 10. However there seems to be no way to specify that the white should be at 1 on this scale.
  Is there a way to do that?</p>
  
  <p>Note again that I need to make use a gradient of colors here and a logarithmic normalization. </p>