title: Clear plot on jupyter notebook with IPython widget before plotting a new graph
body: <p>This is my code. I'd like to click the button plot the graph and then remove the graph plotted before plotting a new graph on clicking the button.</p>
  
  <pre><code>columns=[i for i in load_weekday['feed_point_number'].unique()]
  
  selection=widgets.Dropdown(description='Select feed_point_number?')
  selection.options=columns
  display(selection)
  
  def on_button_clicked(b):
      sns.set_style("whitegrid")
      sns.set_context(rc={"figure.figsize":(12,9)})
      load_weekday.plot(x='time_code',y=selection.value,color='Blue')
  button=widgets.Button(description="Plot")
  display(button)
  
  button.on_click(on_button_clicked)
  </code></pre>