title: How to change specific position's color with matplotlib.pyplot.imshow?
body: <p>I have a numpy array, and I use following codes to draw a simple picture.</p>
  <pre><code>import numpy as np
  from matplotlib import pyplot as plt
  plt.show(image, cmap='gray')
  </code></pre>
  <p>I also have a list containing the several positions of the image, and I'd like to change the colors of these positions on the same picture.
  For example, I have another list like this:</p>
  <pre><code>pos = [(0,1),(3,6)...]
  </code></pre>
  <p>I'd like to change the pixel's color according to this. For other pixels they remain the same.
  How can I do that?</p>