delete duplicates from vertices before passing to voronoi

This fixes the problem where a scatter plot (as well as other types of plots that use scatter, including lines that use scatter to determine the mouse-over areas) generates an error for recent versions of d3 (after about 3.3.7) and where there are overlapping points.

The problem is that `voronoi()` requires the vertices that it's passed to  not contain duplicates. (see: https://github.com/mbostock/d3/wiki/Voronoi-Geom#wiki-_voronoi).

This solves issues #330 and #368/#427. It may be a better fix than the latter as it fixes the underlying cause. I also found cases where 427 did not fix the problem.