Dual y-axis graphing functions

ggplot2, by design, makes it rather difficult to create graphs where the primary and secondary y-axis are on different scales. Hadley Wickham claims this is to prevent people from abusing secondary y-axis to show dodgy correlations (investment banking-style). However, sometimes they have valid uses and the following functions enable this with as little fiddliness as possible.

Usage

sec_rescale_inv(values, scale = 1, shift = 0)

sec_rescale(
  values,
  scale = t61_env$sec_axis_scale,
  shift = t61_env$sec_axis_shift
)

Arguments

values

Vector of data that would normally be passed as the y aesthetic in the graph.

scale

Numeric. Multiplicative factor that rescales the axis. For example, if the scale was originally 0 to 50, then scale = 0.1 would rescale this to 0 to 5.

shift

Numeric. Moves the axis up and down. For example, if the scale was 0 to 5, shift = 5 moves the secondary scale down by 5 units to range from -5 to 0.

Details

sec_rescale_inv is used in the geom that will be displayed on the secondary axis. sec_rescale is used in the sec_axis argument of scale_y_continuous_e61.

See the examples for how to use the two functions to manipulate the secondary axis. Trial and error will be needed to select appropriate scale and shift values.

Note: Due to the way that ggplot functions interact with the R environment, you have to run the code that generates the graph twice after you change it before the changes will show up.

Examples

if (FALSE) {
library(ggplot2)
data <- data.frame(x = 1:5, y1 = 1:5 * 10, y2 = 5:1 - 5)

ggplot(data, aes(x)) +
  geom_col(aes(y = y1)) +
  geom_point(aes(y = sec_rescale_inv(y2, scale = 0.1, shift = 5))) +
  # Some extra arguments are required to correctly format the secondary axis:
  # Set rescaled_sec = TRUE in scale_y_continuous_e61() to format the breaks correctly.
  # The secondary y-axis label (name = "%") needs to be explicitly specified otherwise it will not appear.
  scale_y_continuous_e61(limits = c(0, 60, 10), sec_axis = sec_axis(~sec_rescale(.), name = "%"), rescale_sec = TRUE) +
  labs_e61(y = "%")
}