Time taken to complete graph

This seems like duplication/divergence of date formatting, and potentially confusingly inconsistent.

I think 'return function (d) { return this.format(d, this.formatOptions); }' would be better as it uses standard formatter methods rather than creating ad hoc methods of its own. I think it's reasonable to expect to be able to look at formatters.js to work out how a set of formatting options should behave.