9 Position scales and axes

Every plot has two position scales corresponding to the x and y aesthetics. Typically the user specifies the variables mapped to x and y explicitly, but sometimes an aesthetic is mapped to a computed variable, as happens with geom_histogram(), and does not need to be explicitly specified. For example, the following plot specifications are equivalent:

ggplot(mpg, aes(x = displ)) + geom_histogram()
ggplot(mpg, aes(x = displ, y = after_stat(count))) + geom_histogram()

Although the first example does not state the y-aesthetic mapping explicitly, it still exists and is associated with (in this case) a continuous position scale.