title: Plot a vertical line using matplotlib
body: <p>I would like to draw a vertical line with Matpotlib and I'm using <code>axvline</code>, but it doesn't work. </p>
  
  <pre><code>import sys
  import matplotlib
  matplotlib.use('Qt4Agg')
  
  from ui_courbe import *
  from PyQt4 import  QtGui
  from matplotlib import pyplot as plt
  
  
  class Window(QtGui.QMainWindow, Ui_MainWindow):
      def __init__(self, parent=None):
          super(Window, self).__init__(parent)
          self.setupUi(self) 
          self.boutonDessiner.clicked.connect(self.generatePlot)
  
      def generatePlot(self):
          # generate the plot 
          ax = self.graphicsView.canvas.fig.add_subplot(111)
          ax.plot([1,3,5,7],[2,5,1,-2])
          plt.axvline(x=4)
          self.graphicsView.canvas.draw()
  
  
  if __name__ == '__main__':
      app = QtGui.QApplication(sys.argv)
      win = Window()
      win.show()
      sys.exit(app.exec_())
  </code></pre>
  
  <p>I can see my plot, but no vertical line. Why?</p>