How do you add a general label to facets in ggplot2?

I often have numeric values for faceting. I wish to provide sufficient information to interpret these faceting values in a supplemental title, similar to the axis titles. The labeller options repeat much unnecessary text and are unusable for longer variable titles.

Any suggestions?

The default:

test<-data.frame(x=1:20, y=21:40, facet.a=rep(c(1,2),10), facet.b=rep(c(1,2), each=20))
qplot(data=test, x=x, y=y, facets=facet.b~facet.a)

What I would love:

The best I can do in ggplot:

qplot(data=test, x=x, y=y)+facet_grid(facet.b~facet.a, labeller=label_both)

As indicated by @Hendy, similar to: https://stackoverflow.com/questions/9096671/add-a-secondary-y-axis-to-ggplot2-plots-make-it-perfect

As the latest ggplot2 uses gtable internally, it is quite easy to modify a figure:

library(ggplot2)
test <- data.frame(x=1:20, y=21:40, 
                   facet.a=rep(c(1,2),10), 
                   facet.b=rep(c(1,2), each=20))
p <- qplot(data=test, x=x, y=y, facets=facet.b~facet.a)

# get gtable object
z <- ggplotGrob(p)

library(grid)
library(gtable)
# add label for right strip
z <- gtable_add_cols(z, unit(z$widths[[7]], 'cm'), 7)
z <- gtable_add_grob(z, 
                     list(rectGrob(gp = gpar(col = NA, fill = gray(0.5))),
                          textGrob("Variable 1", rot = -90, gp = gpar(col = gray(1)))),
                     4, 8, 6, name = paste(runif(2)))

# add label for top strip
z <- gtable_add_rows(z, unit(z$heights[[3]], 'cm'), 2)
z <- gtable_add_grob(z, 
                     list(rectGrob(gp = gpar(col = NA, fill = gray(0.5))),
                          textGrob("Variable 2", gp = gpar(col = gray(1)))),
                     3, 4, 3, 6, name = paste(runif(2)))

# add margins
z <- gtable_add_cols(z, unit(1/8, "line"), 7)
z <- gtable_add_rows(z, unit(1/8, "line"), 3)

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

Of course, you can write a function that automatically add the strip labels. A future version of ggplot2 may have this functionality; not sure though.

The secondary axis is now an option: https://ggplot2.tidyverse.org/reference/sec_axis.html

# Basic faceted plot
p <- ggplot(mtcars, aes(cyl, mpg)) +
  geom_point() +
  facet_grid(vs ~ am)
    
    # Create a simple secondary axis for the facets (use the appropriate scale_x function)
p + 
  scale_y_continuous(sec.axis = sec_axis(~ . , name = "SECOND Y AXIS", breaks = NULL, labels = NULL)) +
  scale_x_continuous(sec.axis = sec_axis(~ . , name = "SECOND X AXIS", breaks = NULL, labels = NULL))

There may be a better way to do it, but you can :

fac1 = factor(rep(c('a','b'),10))
fac2 = factor(rep(c('a','b'),10))
data = data.frame(x=1:10, y=1:10, fac1=fac1, fac2=fac2)
p = ggplot(data,aes(x,y)) + ggplot2::geom_point() + 
facet_grid(fac1~fac2)
p + theme(plot.margin = unit(c(1.5,1.5,0.2,0.2), "cm"))
grid::grid.text(unit(0.98,"npc"),0.5,label = 'label ar right', rot = 270) # right
grid::grid.text(unit(0.5,"npc"),unit(.98,'npc'),label = 'label at top', rot = 0)   # top

In addition to the method outlined by kohske, you can add a border to the boxes added by changing

col=NA

to

col=gray(0.5), linetype=1

Also, change

fill=gray(0.5)

for

fill=grey(0.8)

and

gp=gpar(col=gray(1))

to

gp=gpar(col=gray(0))

If you want the new bars to match the facet labels

ie

z <- gtable_add_grob(z, 
      list(rectGrob(gp = gpar(col = gray(0.5), linetype=1, fill = gray(0.8))),
      textGrob("Variable 1", rot = -90, gp = gpar(col = gray(0)))),
      4, 8, 6, name = paste(runif(2)))