How to change color of certain border of strip area of ggplot object?

Question

code of plot:

library(tidyverse)

mtcars %>% 
  ggplot(aes(factor(cyl), disp)) +
  geom_col() +
  facet_grid(. ~ gear, switch = "x") +
  theme(
    strip.placement = "outside",
    strip.background = element_rect(fill = "white", color = "red")
  )

I'm trying to modify the color of certain borders: Expected output:

I tried to change it using grobs but it seems that specific borders cannot be changed.

p <- mtcars %>% 
  ggplot(aes(factor(cyl), disp)) +
  geom_col() +
  facet_grid(. ~ gear, switch = "x") +
  theme(
    strip.placement = "outside",
    strip.background = element_rect(fill = "white", color = "red")
  )

g <- ggplotGrob(p)
g$grobs[[13]]$grobs[[1]]$children$strip.background.x..rect.33641$gp$col

score 1 · Accepted Answer · answered May 11 '21 at 06:20

Based on this answer, you can change the border with linesGrob. In your case you will need polylineGrob because you need two unconnected lines (one on each side of the grids).

Code

p <- mtcars %>% 
  ggplot(aes(factor(cyl), disp)) +
  geom_col() +
  facet_grid(. ~ gear, switch = "x") +
  theme(
    strip.placement = "outside",
    strip.background = element_rect(color = NA)
  )

library(grid)
q <- ggplotGrob(p)

lr2 <-  polylineGrob(x=c(0,0,1,1),
                      y=c(0,1,0,1),
                      id=c(1,1,2,2),
                      gp=gpar(col="blue", lwd=3))

for (k in grep("strip-b",q$layout$name)) {
  q$grobs[[k]]$grobs[[1]]$children[[1]] <- lr2
}

grid.draw(q)

Plot

How to change color of certain border of strip area of ggplot object?

1 Answers1