title: Python pcolormesh with separate alpha value for each bin
body: <p>Lets say I have the following dataset:</p>
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  x_bins = np.arange(10)
  y_bins = np.arange(10)
  z = np.random.random((9,9))
  </code></pre>
  <p>I can easily plot this data with</p>
  <pre><code>plt.pcolormesh(x_bins, y_bins, z, cmap = 'viridis)
  </code></pre>
  <p>However, let's say I now add some alpha value for each point:</p>
  <pre><code>a = np.random.random((9,9))
  </code></pre>
  <p>How can I change the alpha value of each box in the pcolormesh plot to match the corresponding value in array &quot;a&quot;?</p>