Some time ago, I inquired about adding a secondary transformed x-axis in ggplot, and Nate Pope provided the excellent solution described at ggplot2: Adding secondary transfo
As suggested above, you can use sec_axis or dup_axis.
library(ggplot2)
LakeLevels <- data.frame(Day = c(1:365),
Elevation = sin(seq(0, 2*pi, 2 * pi/364)) * 10 + 100)
ggplot(data = LakeLevels) +
geom_path(aes(x = Elevation, y = Day)) +
scale_x_continuous(name = "Elevation (m)", limits = c(75, 125),
sec.axis = sec_axis(trans = ~ . * 3.28084, name = "Elevation (ft)"))
ggplot2 version 3.1.1