It turns out that in order to make this Voronoi library work, I have to reduce the cities' weights so much that they make effectively no difference to the overall figure. But if you'd like a map, then after toying a bit with https://en.wikipedia.org/wiki/File:MapofEmergingUSMegaregions.png and https://en.wikipedia.org/wiki/File:Ninenations.PNG , my un-weighted Voronoi map of North America looks a little like http://www.datapacrat.com/temp/NorAm%20Regions.jpg . Needs some further tweaking, but might be food for thought. :)