How to create a bubble chart as in http://bl.ocks.org/mbostock/raw/4063269/:
...but using NVD3?
The NVD3 site only has reference to Scatter plot http://nvd3.org/ghpages/scatter.html
This custom nvd3 bubble chart implementation might help anyone who wants to figure out how to do it: https://github.com/enplore/nvd3-charts.