Hover not working for heavily overlapped points

[e-koch]

This became apparent when adding in Dario Colombo's dataset if ~1000 GMCs. The large cluster of GMCs in the plot leads to hover not working for most of them. The points on top (ie. that were plotted last) continue to work.

[e-koch]

This may help: http://stackoverflow.com/questions/15554908/d3-js-mouseover-fails-to-trigger-with-an-overlapping-object