hadley/scales — pull request #40 by dougmitarotonda (2014-02-19 16:57:38+00:00)
Title: Change dollar to print negative sign on the outside

I am not sure where the authoritative source on this would be, but how about:
http://english.stackexchange.com/questions/124797/how-to-write-negative-currency-in-text

This commit is to fix https://github.com/hadley/scales/issues/38