title: How to plot gray level image by matplotlib.pyplot.imshow?
body: <p>My data is 2D image in format of numpy.array. By the following code:</p>
  
  <pre><code>fig = plt.figure()
  ax = fig.add_subplot(111)
  ax.imshow(visu_base)
  plt.show()
  </code></pre>
  
  <p>I get a color image from Python. But, how can I show it in gray level?</p>