title: matshow: colormap the values in numpy array to binary
body: <p>I try to visualize a numpy array to image.
  But the array contains negative values or values that out of bound.</p>
  
  <p>I have get an image successfully, but I have some question about gray scale.</p>
  
  <p>How does it interpret through the colormap ?. 
  (In principle, if the array is a float array scaled to 0..1, it should be interpreted as a grayscale image)
  So, what is the principle?</p>
  
  <p>Thank you very much for your collaboration.</p>
  
  <pre><code>V = np.loadtxt('np.txt')
  fig = plt.figure()
  ax = fig.add_subplot(1, 1, 1)       
  ax.matshow(V, cmap = matplotlib.cm.binary)
  plt.show()
  
  The np array like:
  [ [....]
  ....
   [  7.47859700e-03  -4.42994573e-03  -3.15871151e-02   4.57486308e-02
      4.58066400e-02   7.81800849e-02   1.41268438e-01   2.67617603e-01
      3.98583385e-01   3.85877649e-01   1.92501545e-01   2.65159152e-01
      2.10979793e-01   2.48940247e-01   1.75112904e-01  -3.06361785e-02
      2.74774650e-01   1.81465161e-01   4.23131349e-03  -3.56762525e-02
     -1.72089055e-02  -4.25273422e-02  -2.63428158e-02  -4.59487077e-02
     -2.30976482e-02  -4.45129524e-02   8.95580352e-03   1.56548770e-03]
  ...
  [...] ]
  </code></pre>