title: what is 'QtGui.QApplication.setGraphicsSystem("raster")'?
body: <p>when using pyqt, I often get a warning advising me to use the instruction <code>QtGui.QApplication.setGraphicsSystem("raster")</code>. Why and what is 'raster' ?</p>
  
  <p>Also, when should I do it ? in the pyqt doc I read: 'This function is only effective before the QApplication constructor is called'. But python seems happy if I insert it like that:</p>
  
  <pre><code>app = QtGui.QApplication(sys.argv)
  QtGui.QApplication.setGraphicsSystem("raster") # why? just to please a Warning ?
  app.exec_()
  </code></pre>
  
  <hr>
  
  <p>additional information. The warning is:</p>
  
  <pre><code>Warning: QApplication was created before pyqtgraph was imported; there may be problems (to avoid bugs, call QApplication.setGraphicsSystem("raster") before the QApplication is created).
  </code></pre>
  
  <p>I use python 3.5.1 on Mac OS 10.11 with pyqt 4.11.4 and pyqtgraph 0.9.10</p>
  
  <p>When I do not use <code>setGraphicsSystem("raster")</code> my GUI is sometimes not displayed, although it works most of the time. I didn’t see why/where/when it bugs or anything that could help in predicting it, there is no message, just the GUI is not displayed. So I use this instruction just pragmatically.</p>
  
  <p>note: I do not particularly want to use <code>setGraphicsSystem("raster")</code>. It’s just that I am curious of what it means. Even if it seems explicit, I don't quite understand the message, maybe because I don't know what "raster" is.</p>