It is unfortunately not a fairly easy problem. The two main problems are: 1) there is no "documented" way to do this, so you won't find it described in the help. 2) presumably you plot everything with a single plot/line object. This can only trigger an event that says "the mouse is over your data". to figure out WHICH data point it's over, you need to do all the calculations yourself.
I have attached a demo file that i made that shows the basics. For real implementations i like to also add a timer to the windowbuttonmotionfcn so it only runs if the mouse has been stationary for e.g. 0.1 seconds... this prevents popups from going all over the place when you're just moving over data. Also this example seems to do a lot of flashing when i run it in 2014b... not sure why that's happening... it's an old example. I certainly still do this kind of thing in 2014b and it works fine with no flashing, so i'm sure there's a workaround.
