Split facet_wrap panel when x is a factor in ggplot2

Question

I want to add a vertical line to split facet_wrap panel in ggplot2.

ggplot(mpg, aes(drv, hwy))+
  geom_boxplot(aes(colour = drv))+
  facet_wrap(~year)+
  theme_bw()+
  theme(panel.grid=element_blank())

my question is how could we add vertical lines when x is a factor ?

Answer 1

From ?scale_x_discrete :

You can use continuous positions even with a discrete position scale - this allows you (eg) to place labels between bars in a bar chart.

You can add vertical lines with geom_vline() & set the position at 1.5, 2.5, etc:

ggplot(mpg, aes(drv, hwy)) +
  geom_boxplot(aes(colour = drv))+
  geom_vline(xintercept = c(1.5, 2.5), linetype = "dashed") +
  facet_wrap(~ year) +
  theme_bw() +
  theme(panel.grid=element_blank())