Add a coloured border between facets in ggplot2

Question

I wish to add a thick dark grey bar in between each facet. I don't want a border around each facet, instead a dark grey bar in between plots.

library(ggplot2)

ggplot(mpg, aes(cty, hwy, color = factor(year))) + 
  geom_point() +
  facet_wrap(~ cyl, nrow = 1) 

I found this question although my data is as per the example above, only one row and not a facet_grid.

Answer 1

This is based on the not-accepted answer to the question that you linked. (I think the accepted answer is not the best. It adds a coloured border round each of the plot panels and each of the strips.) The number of columns between facet_wrap panels is different from the number of columns between facet_grid panels; hence a slight adjustment for the facet_wrap plot.

library(ggplot2)
library(grid)
library(gtable)

p = ggplot(mpg, aes(cty, hwy, color = factor(year))) + 
  geom_point() +
  facet_wrap(~ cyl, nrow = 1) 

gt <- ggplotGrob(p)


panels = subset(gt$layout, grepl("panel", gt$layout$name), t:r)

# The span of the vertical gap
Bmin = min(panels$t) - 1
Bmax = max(panels$t)

# The columns of the gaps (two to the right of the panels
cols = unique(panels$r)[-length(unique(panels$r))] + 2

# The grob - grey rectangle
g = rectGrob(gp = gpar(col = NA, fill = "grey40"))

## Add greyrectangles into the vertical gaps
gt <- gtable_add_grob(gt, 
      rep(list(g), length(cols)),
      t=Bmin, l=cols, b=Bmax)

## Draw it
grid.newpage()
grid.draw(gt)