title: python matplotlib add and remove text to figure using button events
body: <p>I'm trying to add text to a graph at the location of the mouse pointer  when button_press_event is called and remove it when button_release_event is called.  I have successfully added the text but I can not get it to erase.  Here is part of the code I used:</p>
  
  <pre><code>def onclick(event):
      print 'you pressed', event.button, event.xdata, event.ydata
      plt.text(event.xdata, event.ydata, 'TESTTEST', fontsize=8)
      fig.canvas.draw()
  
  def offclick(event):
      print 'you released', event.button, event.xdata, event.ydata
      #not sure what to put here
      #I tried:
      #plt.text(event.xdata, event.ydata, '')
      fig.canvas.draw()
  
  fig.canvas.mpl_connect('button_press_event', onclick)
  fig.canvas.mpl_connect('button_release_event', offclick) 
  
  plt.show()
  </code></pre>