title: How to draw a colored pixel on a grayscale image?
body: <p>I have a bidimensional ndarray. Each element is an integer. I display the matrix in this way:</p>
  
  <pre><code>plt.figure()
  plt.imshow(img, cmap='Greys')
  </code></pre>
  
  <p>where <strong>img</strong> is the bidimensional array.
  Now I would to highlight some points on the image: for example I would that the pixel in (x,y) is red (I don't want change the real pixel value, but only display the image with the red pixel).</p>
  
  <p>Any suggestion?</p>