Seeking workaround for gtable_add_grob code broken by ggplot 2.2.0

Question

In plots with multiple facet variables, ggplot2 repeats the facet label for the \"outer\" variable, rather than having a single spanning facet strip across all the levels of

Answer 1

Indeed, ggplot2 v2.2.0 constructs complex strips column by column, with each column a single grob. This can be checked by extracting one strip, then examining its structure. Using your plot:

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

# Your data
df = structure(list(location = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 
 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 
 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
 2L, 2L), .Label = c("SF", "SS"), class = "factor"), species = structure(c(1L, 
 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
 2L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("AGR", "LKA"), class = "factor"), 
    position = structure(c(1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 
    2L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 1L, 
    1L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 
    2L), .Label = c("top", "bottom"), class = "factor"), density = c(0.41, 
    0.41, 0.43, 0.33, 0.35, 0.43, 0.34, 0.46, 0.32, 0.32, 0.4, 
    0.4, 0.45, 0.34, 0.39, 0.39, 0.31, 0.38, 0.48, 0.3, 0.42, 
    0.34, 0.35, 0.4, 0.38, 0.42, 0.36, 0.34, 0.46, 0.38, 0.36, 
    0.39, 0.38, 0.39, 0.39, 0.39, 0.36, 0.39, 0.51, 0.38)), .Names = c("location", 
   "species", "position", "density"), row.names = c(NA, -40L), class = "data.frame")

# Your ggplot with three facet levels
p=ggplot(df, aes("", density)) + 
  geom_boxplot(width=0.7, position=position_dodge(0.7)) + 
  theme_bw() +
  facet_grid(. ~ species + location +  position) +
  theme(panel.spacing=unit(0,"lines"),
     strip.background=element_rect(color="grey30", fill="grey90"),
     panel.border=element_rect(color="grey90"),
     axis.ticks.x=element_blank()) +
  labs(x="")

# Get the ggplot grob
pg = ggplotGrob(p)

# Get the left most strip
index = which(pg$layout$name == "strip-t-1")
strip1 = pg$grobs[[index]]

# Draw the strip
grid.newpage()
grid.draw(strip1)

# Examine its layout
strip1$layout
gtable_show_layout(strip1)

One crude way to get outer strip labels 'spanning' inner labels is to construct the strip from scratch:

# Get the strips, as a list, from the original plot
strip = list()
for(i in 1:8) {
   index = which(pg$layout$name == paste0("strip-t-",i))
   strip[[i]] = pg$grobs[[index]]
}

# Construct gtable to contain the new strip
newStrip  = gtable(widths = unit(rep(1, 8), "null"), heights = strip[[1]]$heights)

## Populate the gtable    
# Top row
for(i in 1:2) {
   newStrip = gtable_add_grob(newStrip, strip[[4*i-3]][1], 
           t = 1, l = 4*i-3, r = 4*i)
}

# Middle row
for(i in 1:4){
   newStrip = gtable_add_grob(newStrip, strip[[2*i-1]][2], 
         t = 2, l = 2*i-1, r = 2*i)
}

# Bottom row
for(i in 1:8) {
   newStrip = gtable_add_grob(newStrip, strip[[i]][3], 
       t = 3, l = i)
}

# Put the strip into the plot 
# (It could be better to remove the original strip. 
# In this case, with a coloured background, it doesn't matter)
pgNew = gtable_add_grob(pg, newStrip, t = 7, l = 5, r = 19)

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

OR using vectorised gtable_add_grob (see the comments):

pg = ggplotGrob(p)

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

# Construct gtable to contain the new strip
newStrip  = gtable(widths = unit(rep(1, 8), "null"), heights = strip[[1]]$heights)

## Populate the gtable    
# Top row
cols = seq(1, by = 4, length.out = 2)
newStrip = gtable_add_grob(newStrip, lapply(strip[cols], `[`, 1), t = 1, l = cols, r = cols + 3)

# Middle row
cols = seq(1, by = 2, length.out = 4)
newStrip = gtable_add_grob(newStrip, lapply(strip[cols], `[`, 2), t = 2, l = cols, r = cols + 1)

# Bottom row
newStrip = gtable_add_grob(newStrip, lapply(strip, `[`, 3), t = 3, l = 1:8)

# Put the strip into the plot
pgNew = gtable_add_grob(pg, newStrip, t = 7, l = 5, r = 19)

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

Answer 2

EDIT To allow for panels of different widths (i.e., scales = "free_x", space = "free_x").

This attempt takes the original ggplot, extracts some information, then constructs a new grob containing the overlapping strips. The function is not pretty but it works ... so far. It requires plyr to be installed.

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


df = structure(list(location = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 
    1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
    2L, 2L), .Label = c("SF", "SS"), class = "factor"), species = structure(c(1L, 
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
    1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
    2L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("AGR", "LKA"), class = "factor"), 
        position = structure(c(1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 
        2L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 1L, 
        1L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 
        2L), .Label = c("top", "bottom"), class = "factor"), density = c(0.41, 
        0.41, 0.43, 0.33, 0.35, 0.43, 0.34, 0.46, 0.32, 0.32, 0.4, 
        0.4, 0.45, 0.34, 0.39, 0.39, 0.31, 0.38, 0.48, 0.3, 0.42, 
        0.34, 0.35, 0.4, 0.38, 0.42, 0.36, 0.34, 0.46, 0.38, 0.36, 
        0.39, 0.38, 0.39, 0.39, 0.39, 0.36, 0.39, 0.51, 0.38)), .Names = c("location", 
    "species", "position", "density"), row.names = c(NA, -40L), class = "data.frame")

# Begin with a regular ggplot with three facet levels
p=ggplot(df, aes("", density)) + 
  geom_boxplot(width=0.7, position=position_dodge(0.7)) + 
  theme_bw() +
  facet_grid(. ~ species + location + position) +
  theme(panel.spacing=unit(0,"lines"),
        strip.background=element_rect(color="grey30", fill="grey90"),
        panel.border=element_rect(color="grey90"),
        axis.ticks.x=element_blank()) +
  labs(x="")

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

# Get the ggplot grob
g = ggplotGrob(plot)

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

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

# Number of rows
NumberOfRows = length(strip[[1]])

# Panel spacing and it's unit
plot_theme <- function(p) {
   plyr::defaults(p$theme, theme_get())
}
PanelSpacing = plot_theme(plot)$panel.spacing
unit = attr(PanelSpacing, "unit")

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

  for(j in 1:NumberOfStrips) {
   Nlabel[[i]][j] = getGrob(grid.force(strip[[j]][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
# Set the widths of the strips, based on widths of the panels and PanelSpacing
panel = subset(g$layout, grepl("panel", g$layout$name), l, drop = TRUE)                       
StripWidth = list()
for(i in seq_along(panel)) StripWidth[[i]] = unit.c(g$width[panel[i]], PanelSpacing)

newStrip  = gtable(widths = unit.c(unit(unlist(StripWidth), c("null", unit)))[-2*NumberOfStrips], 
                   heights = strip[[1]]$heights)


## Populate the gtable  
seqLeft = list()
for(i in 1:NumberOfRows) {  
   Left = which(map[[i]] == TRUE)
   seqLeft[[i]] = if((i-1) < 1) 2*Left - 1 else sort(unique(c(seqLeft[[i-1]], 2*Left - 1))) 
   seqRight = c(seqLeft[[i]][-1] -2, (2*NumberOfStrips-1))
   newStrip = gtable_add_grob(newStrip, lapply(strip[(seqLeft[[i]]+1)/2], `[`, i), t = i, l = seqLeft[[i]], r = seqRight)
}

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

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

return(pgNew)
}

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

It probably would not be too difficult to break the function, but I tried it on data where the sequencing of the rows is not so even.

p1 = ggplot(mtcars, aes("", hp)) + 
  geom_boxplot(width=0.7, position=position_dodge(0.7)) + 
  theme_bw() +
  facet_grid(. ~ vs + am + carb, labeller = label_both) +
  theme(panel.spacing=unit(0.2,"lines"),
        strip.background=element_rect(color="grey30", fill="grey90"),
        panel.border=element_rect(color="grey90"),
        axis.ticks.x=element_blank()) +
  labs(x="")

  grid.draw(OverlappingStripLabels(p1))

p2 = ggplot(mtcars, aes("", hp)) + 
  geom_boxplot(width=0.7, position=position_dodge(0.7)) + 
  theme_bw() +
  facet_grid(. ~ vs + carb +  am, labeller = label_both) +
  theme(panel.spacing=unit(0.2,"lines"),
        strip.background=element_rect(color="grey30", fill="grey90"),
        panel.border=element_rect(color="grey90"),
        axis.ticks.x=element_blank()) +
  labs(x="")

 grid.draw(OverlappingStripLabels(p2))




df = structure(list(id = 1:19, 
category1 = c("X", "X", "X", "X", "X", "X", "X", "X", "X", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y"),
category2 = c(21L, 21L, 21L, 22L, 22L, 22L, 22L, 22L, 22L, 23L, 23L, 23L, 24L, 24L, 24L, 25L, 25L, 26L, 26L), 
category3 = c("C1", "C2", "C3", "D1", "D2", "D3", "D5", "D6", "D7", "E1", "E2", "E3", "F1", "F2", "F3", "G1", "G2", "H1", "H2"), 
freq = c(4L, 7L, 4L, 28L, 20L, 0L, 1L, 4L, 1L, 17L, 33L, 31L, 20L, 20L, 21L, 15L, 18L, 12L, 13L)), 
.Names = c("id", "category1", "category2", "category3", "freq"), class = "data.frame", row.names = c(NA, -19L))

p3 = ggplot(df, aes(category3, freq)) + 
  geom_bar(stat = "identity") + 
  facet_grid(. ~ category1 + category2, scale = "free_x", space = "free_x")

 grid.draw(OverlappingStripLabels(p3))