Starting with ggplot2 2.2.0 you can add a secondary axis like this (taken from the ggplot2 2.2.0 announcement):
ggplot(mpg, aes(displ, hwy)) +
geom_point() +
scale_y_continuous(
"mpg (US)",
sec.axis = sec_axis(~ . * 1.20, name = "mpg (UK)")
)