r - Annotating facet title as strip over facet

Question

I want to add a facet title as strip over a facetted plot in ggplot2. My MWE throws an error.

library(ggplot2)
library(gtable)
p <- ggplot(mtcars, aes(mpg, wt)) + geom_point()
p <- p + facet_grid(. ~ cyl)


# get gtable object
Plot1 <- ggplot_gtable(ggplot_build(p))
# add label for top strip
Plot1 <- gtable_add_rows(Plot1, Plot1$heights[[3]], 2)
Plot1 <- gtable_add_grob(Plot1,
  list(rectGrob(gp = gpar(col = NA, fill = gray(0.5))),
  textGrob("Cyl", gp = gpar(col = gray(1)))),
  3, 4, 3, 10, name = paste(runif(2)))

# add margins
Plot1 <- gtable_add_rows(Plot1, unit(1/8, "line"), 2)

# draw it
grid.newpage()
print(grid.draw(Plot1))

Answer 1

Edit Updating to ggplot version 3.0.0

I think you were close. You had the row and column numbers wrong in the add grob command. Your new strip spans columns 5 to 9; that is, change 10 to 9 in the add grob command.

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

p <- ggplot(mtcars, aes(mpg, wt)) + geom_point()
p <- p + facet_grid(. ~ cyl)

z <- ggplotGrob(p)

#  New strip at the top
z <- gtable_add_rows(z, z$height[7], pos = 6)  # New row added below row 6

# Check the layout
gtable_show_layout(z)   # New strip goes into row 7 
                        # New strip spans columns 5 to 9

z <- gtable_add_grob(z, 
  list(rectGrob(gp = gpar(col = NA, fill = "gray85", size = .5)),
       textGrob("Number of Cylinders", gp = gpar(cex = .75, fontface = 'bold', col = "black"))), 
  t=7, l=5, b=7, r=9, name = c("a", "b"))

# Add small gap between strips - below row 6
z <- gtable_add_rows(z, unit(2/10, "line"), 7)

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

OR, to add a new strip to the right:

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

p <- ggplot(mtcars, aes(mpg, wt)) + geom_point()
p <- p + facet_grid(cyl ~ .)

z <- ggplotGrob(p)

#  New strip to the right
z <- gtable_add_cols(z, z$widths[6], pos = 6)  # New column added to the right of column 6

# Check the layout
gtable_show_layout(z)   # New strip goes into column 7 
                        # New strip spans rows 7 to 11
z <- gtable_add_grob(z, 
  list(rectGrob(gp = gpar(col = NA, fill = "gray85", size = .5)),
       textGrob("Number of Cylinders", rot = -90,  
          gp = gpar(cex = .75, fontface = 'bold', col = "black"))), 
  t=7, l=7, b=11, r=7, name = c("a", "b"))

# Add small gap between strips - to the right of column 6
z <- gtable_add_cols(z, unit(2/10, "line"), 6)

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