title: Two different color colormaps in the same imshow matplotlib
body: <p>Let's suppose the example below</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import numpy as np
  
  v1 = -1 + 2*np.random.rand(50,150)
  fig = plt.figure()
  ax = fig.add_subplot(111)
  p = ax.imshow(v1,interpolation='nearest')
  cb = plt.colorbar(p,shrink=0.5)
  plt.xlabel('Day')
  plt.ylabel('Depth')
  cb.set_label('RWU')
  plt.show()
  </code></pre>
  
  <p>I want to show the values below zero in a different colormap than the values above zero </p>