Optimize canvas point interactions

[lindsayplatt]

The find_closest_point function isn't working on zoom, plus kind of lags behind your mouse. Wondering if there's a better way to implement it? Potentially this: http://bl.ocks.org/Jverma/70f7975a72358e6d69cdd4bf6a0569e7

[lindsayplatt]

Marty likely has a completely different and better way for interactions with canvas. Complete delete find_closest_point and do something different if that makes more sense.