title: Limit bokeh plot pan to defined range
body: <p>I was wondering if it were possible to limit the range of the "pan" tool for bokeh generated plots? For example, say I had this simple plot:</p>
  
  <pre><code>from bokeh.plotting import output_file, rect, show
  output_file('test.html')
  rect([10,20,30], [10,20,30], width=[1,2,3], color=['red','blue','green'], height=5, plot_width=400, plot_height=400, tools = "ypan,box_zoom,reset")
  show()
  </code></pre>
  
  <p>The ypan tool works great, but I could keep panning until my graph disappears. Is there any way I can constrain the pan?</p>