Add "background" to axis title

axis-bg.R

library(ggplot2)
library(grid)

myGrob <- grobTree(rectGrob(gp = gpar(fill = "purple", alpha = 0.5)))

ggplot(iris, aes(x = Sepal.Length, y = Petal.Length)) +
  geom_point() +
  annotation_custom(myGrob, xmin = -10, xmax = -1, ymin = -10, ymax = Inf) + # x axis
  annotation_custom(myGrob, xmin = -10, xmax = Inf, ymin = -10, ymax = -1) + # y axis
  coord_cartesian(xlim = c(0, 10), ylim = c(0, 10), clip = "off") +
  theme(
    plot.margin = margin(0, 0, 100, 100)
  ) +
  ggsave("~/Desktop/axis-bg.png")