Function for adding units to axis labels

ggplot_axis_units.R

library(tidyverse)

add_units <- function(p, unit, scale = "x", outside = TRUE) {
  
  params <- ggplot_build(p)$layout$panel_params[[1]]
  
  if(scale == "x") {
    brks <- params$x.major_source 
    scale <- scale_x_continuous
  } else {
    brks <- params$y.major_source
    scale <- scale_y_continuous
  }
  
  idx <- if(outside) length(brks) else 1
  
  names(brks) <- brks
  names(brks)[idx] <- paste(brks[idx], unit)
  suppressMessages(p + scale(breaks = brks))
}

p <- ggplot(iris, aes(x = Petal.Length, y = Sepal.Width, color = Species)) +
  geom_point() +
  theme(axis.text.x = element_text(hjust = 0)) +
  labs(x = "Petal Length", y = "Sepal Length", title = "Iris")

p %>% 
  add_units("cm", "x") %>% 
  add_units("cm", "y")