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

Question

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: add a secondary y axis to ggplot2 plots - make it perfect

Answer 1

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.

Answer 2

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))