title: Matplotlib + Qt: tight_layout() doesn't work on the first draw
body: <p>Using <code>PyQt4</code> and <code>matplotlib</code> I have connected a button click to perform some calculation and render a graph. The <code>tight_layout()</code> only applies after clicking the button a second time.</p>
  
  <p>When I'm done setting up the axes and putting data on the graph, I call</p>
  
  <pre><code>fig.tight_layout()
  fig.canvas.draw()
  </code></pre>
  
  <p>I've tried to fake a second button press without success:</p>
  
  <pre><code>from PyQt4 import QtGui
  QtGui.QApplication.processEvents()
  fig.tight_layout()
  fig.canvas.draw()
  QtGui.QApplication.processEvents()
  fig.tight_layout()
  fig.canvas.draw()
  </code></pre>
  
  <p>My thought was that the Qt surface wasn't recognized as dirty, but resizing the window redraws the chart with the same loose layout. The tight layout does apply when I clear the axes and repopulate the chart.</p>
  
  <p>How can I make <code>tight_layout()</code> apply the first time the graph is drawn?</p>