title: matplotlib imshow() using 3xN RGB array
body: <p>I use custom 3xN data with value range (0, 1) as RGB color, and want to use matplotlib.imshow() to show it.</p>
  
  <pre><code>import pylab as plt
  import numpy as np
  
  Z = np.vstack([np.zeros((1, 256)), np.zeros((1, 256,)), np.zeros((1, 256,))])
  im = plt.imshow(Z, interpolation='none', aspect='auto')
  plt.colorbar(im, orientation='horizontal')
  plt.show()
  </code></pre>
  
  <p>I expect this to give me a black image. But instead I get a green one like this:
  <img src="https://i.stack.imgur.com/bs12X.png" alt="enter image description here"></p>
  
  <p>And the Y-ticks look funny. I don't understand the -0.5 tick on the Y-axis at all. Why does the y-axis range between [-0.5, 2.5]?</p>