title: How to update the contents of a FigureCanvasTkAgg
body: <p>I'm plotting some data in a Tkinter FigureCanvasTkagg using <code>matplotlib</code>. 
  I need to clear the figure where I plot data and draw new data when a button is pressed.</p>
  
  <p>Here is the plotting part of the code (there's an App class defined before):</p>
  
  <pre><code>    self.fig = figure()
      self.ax = self.fig.add_subplot(111)
      self.ax.set_ylim( min(y), max(y) )      
  
      self.line, = self.ax.semilogx(x, y, '.-')   #tuple of a single element
      self.canvas = FigureCanvasTkAgg(self.fig, master=master)
      self.ax.semilogx(x, y, 'o-')
      self.canvas.show()
      self.canvas.get_tk_widget().pack(side='top', fill='both', expand=1)
      self.frame.pack()   
  </code></pre>
  
  <p>How do I update the contents of such a canvas?</p>