title: Multiple titles (suptitle) with subplots
body: <p>I have a series of 9 subplots in a 3x3 grid, each subplot with a title. 
  I want to add a title for each row. To do so I thought about using suptitle.
  The problem is if I use 3 suptitles they seems to be overwritten and only the last one seems to be shown.</p>
  
  <p>Here is my basic code:</p>
  
  <pre><code>fig, axes = plt.subplots(3,3,sharex='col', sharey='row')
  
  for j in range(9):
      axes.flat[j].set_title('plot '+str(j))
  
  plt1 = fig.suptitle("row 1",x=0.6,y=1.8,fontsize=18)
  plt2 = fig.suptitle("row 2",x=0.6,y=1.2,fontsize=18)
  plt3 = fig.suptitle("row 3",x=0.6,y=0.7,fontsize=18)
  fig.subplots_adjust(right=1.1,top=1.6)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/6LSmr.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/6LSmr.png" alt="enter image description here"></a></p>