Allow setting just one of the limits of scale

[wch]

It would be handy to be able to set just the lower or upper limit of a scale. For example, when making a line graph, you might just want to set the lower y limit to 0, but let ggplot automatically calculate the upper bound.

# Lower y value on plot is 8
ggplot(BOD, aes(Time, demand)) + geom_line()

# Something like this would be useful
ggplot(BOD, aes(Time, demand)) + geom_line() + ylim(0,NULL)

[hadley]

This sounds like a great feature, but unfortunately we don't currently have the development bandwidth to support it. If you'd like to submit a pull request that implements this feature, please follow the instructions in the development vignette.