title: matplotlib slider redraw does not update figtext
body: <p>I used sliders in matplotlib to update a few graphs based on GUI input. 
  All my graphs update well.
  But when I use figtext, I have the problem that the updated text will write over the existing one.</p>
  
  <pre><code>import numpy as np
  import pylab as p
  from matplotlib.widgets import Slider
  
  p.subplot(111) 
  x = np.arange(0,500,1)
  f = np.sin(x/100.0)
  l11, = p.plot(f)
  
  ax = p.axes([0.25, 0.05, 0.7, 0.03], axisbg='lightgoldenrodyellow')
  slider1 = Slider(ax, 'amplitude', -1.0, 1.5, valinit=0)
  
  def update(val):
      f = slider1.val * np.sin(x/100.0)
      l11.set_ydata(f)
      np.set_printoptions(precision=2)
      p.figtext(0.5, 0.65,  str(slider1.val) )
      p.draw()
  
  slider1.on_changed(update)
  p.show()
  </code></pre>