d3.sum.

[mbostock]

But, d3.stats.mean and d3.stats.median.

[jasondavies]

These are already in #143, although mean and median are in d3.stats. I can pull them out into a separate pull request for core if you'd like?

[mbostock]

I'm thinking only d3.sum should be included in core. Not because it's a statistic per se, but because it's needed to do layout of things like stacked bars. Also, for consistency with Python, note that Python has sum() built-in but not the others.

If you wouldn't mind breaking that out as a separate pull request, that'd be great! I'm still for having a stats module, but I should probably review the API for it in a bit more detail before we commit to not making backwards-compatible changes.

Also, have you seen https://github.com/nsfmc/atoll.js by @nsfmc?