ggplot2における2軸プロットの使いみち ref: http://qiita.com/nozma/items/c50a8d45c73bf8b84794

file0.r

library(ggplot2)
library(dplyr)

# hwy : highway miles per gallon
#  1mile == 1.60934km
#  1gallon == 3.78541L
#  ∴ mile / gallon = 1.60934 / 3.78541 km / L

mpg2kpl <- function(x) x * 1.60934 / 3.78541

mpg %>% 
  ggplot(aes(displ, hwy)) +
  geom_point() +
  scale_y_continuous(sec.axis = sec_axis(~ mpg2kpl(.), name = "km/L")) +
  labs(y = "mile/gal") +
  theme_classic()

file1.r

# temperature: deg C
# pressure:    mm of Hg
# 1 mmHg == 0.133322 kPa

mmHg2kPa <- function(x) x * .133322
C2K <- function(x) x + 273.15

pressure %>%
  ggplot(aes(temperature, pressure)) +
  geom_point() +
  scale_x_continuous(sec.axis = sec_axis(~ C2K(.), name = "Temperature (K)")) +
  scale_y_continuous(sec.axis = sec_axis(~ mmHg2kPa(.), name = "Pressure (kPa)")) +
  labs(x = "Temperature (C)",  y = "Pressure (mm of Hg)") +
  theme_classic()