hadley/ggplot2 — pull request #684 by jimhester (2012-10-10 15:43:03+00:00)
Title: Allow setting one of the limits of scale

Implements setting of only one limit of a continuous scale by specifying
NA for the limit which you don't want to set. See #557.

I had to use NA instead of NULL to implement this, as is.null() is not
vectorized, a vector with null at any location is TRUE.