Making a second axis on ggplot

two-axis-example.R

library(tidyverse)

# "faithful" is a dataset about a geyser, showing size of eruption vs. waiting
# time before the eruption
data <- as_tibble(datasets::faithful) %>%
  mutate(index = 1:n()) %>%
  head(10)

data

# values for "eruptions" range from 2 to 5; for "waiting", from 50 to 90
data %>%
  pivot_longer(!index) %>%
  ggplot(aes(index, value)) +
  facet_grid(rows = vars(name), scales = "free_y") +
  geom_line()

# using ggplot -----------------------------------------------------------------

# get a scaling factor that is median of eruptions:waiting ratio
scale_factor <- with(data, { median(eruptions / waiting) })

data %>%
  # scale the value to be shown on the plot (which always goes with main y-axis)
  mutate(scaled_waiting = waiting * scale_factor) %>%
  select(index, eruptions, waiting = scaled_waiting) %>%
  pivot_longer(!index) %>%
  ggplot(aes(index, value)) +
  geom_line(aes(color = name)) +
  # the secondary axis is the main axis but *divided* by the scaling factor
  scale_y_continuous(
    "eruptions",
    sec.axis = sec_axis(
      trans = ~ . / scale_factor,
      name = "waiting"
    )
  )

# using cowplot ----------------------------------------------------------------
library(cowplot)

p1 <- data %>%
  ggplot(aes(index, eruptions)) +
  geom_line(color = "red")

p2 <- data %>%
  ggplot(aes(index, waiting)) +
  geom_line(color = "blue") +
  scale_y_continuous(position = "right")

aligned_plots <- align_plots(p1, p2, align = "hv")

ggdraw(aligned_plots[[1]]) +
  draw_plot(aligned_plots[[2]])