title: Bokeh: How to click and drag to display displacement between points
body: <p>I want to be able to click and drag between two points on my graph and display the change in position between points and display a line connecting the two points. How can I go about approaching this?</p>
  <p><a href="https://i.stack.imgur.com/pylJ6.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/pylJ6.png" alt="enter image description here" /></a></p>