Overall Label for Facets

Question

Here is some minimal code to generate a graph with two sets of facets.

library(\"ggplot2\", quietly = TRUE, warn.conflicts = FALSE)
library(\"RColorBrewer\",         


        

Answer 1

This is fairly general. The current locations of the top and right strips are given in the layout data frame. This solution uses those locations to position the new strips. The new strips are constructed so that heights, widths, background colour, and font size and colour are the same as in current strips. There are some explanations below.

# Packages
library(ggplot2)
library(RColorBrewer)
library(grid)
library(gtable)

# Data
val.a <- rnorm(20)
val.b <- rnorm(20)
val.c <- c("A","B","C","D","E","F","G","H","I","J")
val.d <- c("A","B","C","D","E","F","G","H","I","J")
val.e <- rnorm(20)
maya <- data.frame(val.a,val.b,val.c,val.d,val.e)

# Base plot
p <- ggplot(maya, aes(x = val.a, y = val.b)) + 
   geom_point(shape = 20,size = 3, aes(colour = val.e)) + 
   facet_grid(val.c ~ val.d) + 
   xlab("Leonardo") + ylab("Michaelangelo") + 
   scale_colour_gradientn(colours = brewer.pal(9,"YlGnBu"), name = "Splinter")

# Labels 
labelR = "Variable 1"
labelT = "Varibale 2"

# Get the ggplot grob
z <- ggplotGrob(p)

# Get the positions of the strips in the gtable: t = top, l = left, ...
posR <- subset(z$layout, grepl("strip-r", name), select = t:r)
posT <- subset(z$layout, grepl("strip-t", name), select = t:r)

# Add a new column to the right of current right strips, 
# and a new row on top of current top strips
width <- z$widths[max(posR$r)]    # width of current right strips
height <- z$heights[min(posT$t)]  # height of current top strips

z <- gtable_add_cols(z, width, max(posR$r))  
z <- gtable_add_rows(z, height, min(posT$t)-1)

# Construct the new strip grobs
stripR <- gTree(name = "Strip_right", children = gList(
   rectGrob(gp = gpar(col = NA, fill = "grey85")),
   textGrob(labelR, rot = -90, gp = gpar(fontsize = 8.8, col = "grey10"))))

stripT <- gTree(name = "Strip_top", children = gList(
   rectGrob(gp = gpar(col = NA, fill = "grey85")),
   textGrob(labelT, gp = gpar(fontsize = 8.8, col = "grey10"))))

# Position the grobs in the gtable
z <- gtable_add_grob(z, stripR, t = min(posR$t)+1, l = max(posR$r) + 1, b = max(posR$b)+1, name = "strip-right")
z <- gtable_add_grob(z, stripT, t = min(posT$t), l = min(posT$l), r = max(posT$r), name = "strip-top")

# Add small gaps between strips
z <- gtable_add_cols(z, unit(1/5, "line"), max(posR$r))
z <- gtable_add_rows(z, unit(1/5, "line"), min(posT$t))

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