title: matplotlib pcolormesh grid not visible
body: <p>Does anyone know why my grid is not plotted on top of the colormap here.</p>
  
  <pre><code>import matplotlib.pylab as plt
  import numpy as np
  
  Style = 'ggplot'
  plt.style.use([Style])
  
  data = np.random.random((40,40))
  
  fig = plt.figure()
  ax = fig.add_subplot(111)
  ax.pcolormesh(data, cmap=plt.cm.viridis, zorder=1)
  ax.grid(True, zorder=10)
  </code></pre>