Destroying d3 event listeners
We can't let our event listeners hang around forever - we learn how to clean up after ourselves and cancel old ones.
Before we look at adding events to our charts, let's learn how to destroy our event handlers. Removing old event listeners is important for updating charts and preventing memory leaks, among other things.
Let's add a 3 second timeout at the end of our code so we can test that our mouse events are working before we destroy them.
Removing a d3 event listener is easy — all we need to do is call
null as the triggered function.