title: How to mark cells in matplotlib.pyplot.imshow (drawing cell borders)
body: <p>I have a small 2d vector to display:</p>
  
  <pre class="lang-py prettyprint-override"><code>import matplotlib.pyplot as plt
  import numpy as np
  
  img = np.random.rand(4,10)
  plt.imshow(img, cmap='Reds')
  </code></pre>
  
  <p>As folllow:</p>
  
  <p><a href="https://i.stack.imgur.com/ZWTQS.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ZWTQS.png" alt="enter image description here"></a></p>
  
  <p>But now I want to mark a specific cell, in order to focus the reader on that cell. Because this is of specific interest...</p>
  
  <p>Therefore something like a border of this cell would be nice:</p>
  
  <p><a href="https://i.stack.imgur.com/llkFb.png" rel="noreferrer"><img src="https://i.stack.imgur.com/llkFb.png" alt="enter image description here"></a></p>
  
  <p>Does someone know how to archive this with <code>matplotlib</code> in a convenient way?</p>