[Feature Request] Display graph x,y coordinates under the mouse cursor.

[ckgt]

Thanks for contributing to PlotJuggler. You are great! Really!

Problem description

I can't see the x or y graph coordinates when I move my mouse over a graph (like you can in matplotlib). Maybe I'm dumb and just can't figure this out but it would be nice to make it a default if the capability is there, or add the capability. I imagine a nice place for the info would be on the left side of each graph's title bar.

Steps to reproduce (important)

• Move mouse over any graph. Can't see any indication of graph x,y coordinates displayed anywhere.