title: Matplotlib Symmetric Logarithmic colormap not centered at zero
body: <p>I was looking at an issue where the default colormap, <code>SymLogNorm</code> imported from <code>matplotlib.colors</code>, isn't actually symmetric for my data. My data's mean and median are above zero, but I want values of zero to appear as the color white. <code>SymLogNorm</code> returns a colormap with center value very slightly above zero -- subtle, but definitely noticeable.</p>
  <p>Is there any solution to this problem?</p>