title: How to draw only some lines in MATLAB's pcolor plot?
body: <p>I have a block matrix that I visualize with <code>pcolor</code>. I'd like to somehow visualize the block structure, so I'm looking for a mixture between <code>shading flat</code> inside each block and <code>shading faceted</code> at the borders, something like</p>
  
  <pre><code>a | b c | d e f
  - + - - + - - -
  g | h i | j k l
  m | n o | p q r
  </code></pre>
  
  <p>(where each letter represents a color value and the lines separate the blocks)
  How can this be achieved?</p>