title: add a tooltip to 2graph vis.js library
body: <p>I am using a 2d line graph of vis.js (<a href="http://visjs.org/graph2d_examples.html" rel="nofollow">http://visjs.org/graph2d_examples.html</a> ). Is there any way to add a tooltip to the data points so that when you mouse over or click you see the value in a pop up or somewhere else?</p>