hadley/ggplot2 — pull request #594 by kohske (2012-06-25 16:31:10+00:00)
Title: colour bar guides can correctly handle the expression labels

Passed all unit test and probably also visual test.
see this thread: https://groups.google.com/group/ggplot2/browse_thread/thread/3a9efa6d01b8500e

but maybe the xxx_format should return the list of expression instead of list of call.