Normalizing y-axis in histograms in R ggplot to proportion

As of the latest and greatest ggplot2 version 3.0.0, the format has changed. Now you can wrap the y value in stat() rather than messing with .. stuff.

ggplot(mydataframe, aes(x = value)) +
  geom_histogram(aes(y = stat(count / sum(count))))

Note that ..ncount.. rescales to a maximum of 1.0, while ..count.. is the non scaled bin count.

ggplot(mydataframe, aes(x=value)) +
  geom_histogram(aes(y=..count../sum(..count..)))

Which gives: