Adding tooltip interactivity to charts is one of the most desired features,
but also one of the tricker ones to implement. One common solution is to attach
.on('mouseover') event handlers to each circle element. This will work okay,
except when points start overlapping. You will also start using up lots of memory
if the number of points is very large.
A more advanced technique is to use d3.geom.voronoi. This solves the problem
of overlapping points, but you still run into performance issues. That's because