extend ggplot `geom_ribbon()` in the x direction

Question

library(tidyverse)
mQ <- quantile(mtcars$wt, c(0.025, 0.975))
mtcarsQ <- data.frame(x = c(min(as.numeric(diamonds$cut)), 
                            max(as.numeric(diamonds$cut))), 
                      ymin = rep(mQ[1], 2), 
                      ymax = rep(mQ[2], 2))

ggplot() + 
  geom_blank(data = diamonds, aes(x = cut, y = y)) +
  geom_ribbon(data = mtcarsQ, aes(x = x, ymin = ymin, ymax = ymax), alpha=0.2) +
  geom_boxplot(data = diamonds, aes(x = cut, y = y, fill = cut, group = cut)) +
  coord_cartesian(ylim = c(0, 12)) + 
  theme_bw()

I'd like to extend my geom_ribbon() from code chunk above in either direction on the x-axis. Something resembling the image below. My preferred geom_ribbon() would fully entend into the blue dotted line box.

How do I do this?

Answer 1

You could subtract/add 0.5 from mtcarsQ$x

mtcarsQ <- data.frame(x = c(min(as.numeric(diamonds$cut)) - 0.5,
                            max(as.numeric(diamonds$cut)) + 0.5),
                      ymin = rep(mQ[1], 2),
                      ymax = rep(mQ[2], 2))

ggplot() +
  geom_blank(data = diamonds, aes(x = cut, y = y)) +
  geom_ribbon(data = mtcarsQ, aes(x = x, ymin = ymin, ymax = ymax), alpha=0.2) +
  geom_boxplot(data = diamonds, aes(x = cut, y = y, fill = cut, group = cut)) +
  coord_cartesian(ylim = c(0, 12)) +
  theme_bw()

---

Update

In response to your comment, here is an example

mtcars %>%
    mutate(cyl = factor(cyl)) %>%
    ggplot() +
    geom_col(aes(cyl, mpg)) +
    geom_rect(
        data = data.frame(
            xmin = min(as.integer(as.factor(mtcars$cyl))) - 0.5,
            xmax = max(as.integer(as.factor(mtcars$cyl))) + 0.5,
            ymin = 20,
            ymax = 120),
        aes(xmin = xmin, xmax = xmax, ymin = ymin, ymax = ymax),
        alpha = 0.2, fill = "green")