ggplot2: have common facet bar in outer facet panel in 3-way plot

Question

I have the following code:

label_rev <- function(labels, multi_line = TRUE, sep = \": \") {
     label_both(rev(labels), multi_line = multi_line, sep = se         


        

Answer 1

Based on this answer, but sufficiently different to warrant an answer of its own. Given a ggplot with multiple facets on the right margin, this answer provides a function, OverlappingStripLabels(), that takes information from the ggplot to reconstruct the strip so that the labels are overlapping. It uses gtable and grid functions to do so.

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

# Initial plot
plot = ggplot(data = mtcars, aes(wt, mpg)) + geom_point() +
   facet_grid(vs + cyl ~ gear, labeller = label_both) + 
   theme_bw() +
   theme(panel.spacing=unit(.2,"lines"),
         strip.background=element_rect(color="grey30", fill="grey90"))


## The function to get overlapping strip labels
OverlappingStripLabels = function(plot) {

# Get the ggplot grob
pg = ggplotGrob(plot)

### Collect some information about the strips from the plot
# Get a list of strips
strip = lapply(grep("strip-r", pg$layout$name), function(x) {pg$grobs[[x]]})

# Number of strips
NumberOfStrips = sum(grepl(pattern = "strip-r", pg$layout$name))

# Number of columns
NumberOfCols = length(strip[[1]])

# Panel spacing
plot_theme <- function(p) {
   plyr::defaults(p$theme, theme_get())
}
PanelSpacing = plot_theme(plot)$panel.spacing

# Map the boundaries of the new strips
Nlabel = vector("list", NumberOfCols)
map = vector("list", NumberOfCols)
for(i in 1:NumberOfCols) {

  for(j in 1:NumberOfStrips) {
   Nlabel[[i]][j] = getGrob(grid.force(strip[[j]]$grobs[[i]]), gPath("GRID.text"), grep = TRUE)$label
  }

map[[i]][1] = TRUE
for(j in 2:NumberOfStrips) {
   map[[i]][j] = Nlabel[[i]][j] != Nlabel[[i]][j-1]
   }
}

## Construct gtable to contain the new strip
newStrip  = gtable(heights = unit.c(rep(unit.c(unit(1, "null"), PanelSpacing), NumberOfStrips-1), unit(1, "null")), 
                   widths = strip[[1]]$widths)

## Populate the gtable  
seqTop = list()
for(i in NumberOfCols:1) {  
   Top = which(map[[i]] == TRUE)
   seqTop[[i]] = if(i == NumberOfCols) 2*Top - 1 else  sort(unique(c(seqTop[[i+1]], 2*Top - 1)))  
   seqBottom = c(seqTop[[i]][-1] -2, (2*NumberOfStrips-1))
   newStrip = gtable_add_grob(newStrip, lapply(strip[(seqTop[[i]]+1)/2], function(x) x[[1]][[i]]), l = i, t = seqTop[[i]], b = seqBottom)
}

## Put the strip into the plot
# Get the locations of the original strips
pos = subset(pg$layout, grepl("strip-r", pg$layout$name), t:r)

## Use these to position the new strip
pgNew = gtable_add_grob(pg, newStrip, t = min(pos$t), l = unique(pos$l), b = max(pos$b))

return(pgNew)
}

## Draw the plot
grid.newpage()
grid.draw(OverlappingStripLabels(plot))