How can I use grid to edit a ggplot2 object to add math expressions to facet labels?

Question

I need to put Greek letters into facet labels using facet_wrap() in ggplot2. I found a Link describing the same for facet_grid(). I applied this for my data, using the following code:

levels(parameters) <- c(expression(alpha), expression(beta))  
p + facet_grid(.~parameters, labeller = label_parsed)

This works great and does exactly what I want. However, I need to use facet_wrap() instead (to get separate y-axes for both paramters, and also to plot even more parameters in different columns and rows). I tried the following:

p + facet_wrap(.~parameters, labeller = label_parsed)  ,  or  
p + facet_wrap(.~parameters)

but this didn't work because there is no "labeller" function in facet_wrap. How could this be done using grid?

Scrawly

This thread on the ggplot2 mailing list suggests this feature isn't supported (unless you know how to delve into grid objects). That was a while back (Dec 2009) but I'm fairly certain it's still unresolved. – Swinish

Thanks! Do you have a suggestion about what grid-function I may look at to get access to the facet labels? Again, thanks much for your help! – Scrawly

I don't, sorry, I really need to learn more about grid myself. But there are definitely folks around here (or the ggplot2 mailing list) who'd know. You might try editing this question (or asking a new one) to be more like: 'How can I use grid to edit a ggplot2 object to add math expressions to facet labels?'. – Swinish

Answer 1

This example should get you started:

library("ggplot2")
library("grid")

d <- ggplot(iris, aes(Sepal.Length, Sepal.Width)) +
            geom_point() +
            facet_wrap(~Species)
grob <- ggplotGrob(d)
strip_elem <- grid.ls(getGrob(grob, "strip.text.x", grep=TRUE, global=TRUE))$name

grob <- grid::editGrob(grob, strip_elem[1], label=expression(alpha[1]))
grob <- grid::editGrob(grob, strip_elem[2], label=expression(beta^2))
grob <- grid::editGrob(grob, strip_elem[3], label=expression(hat(gamma)))

grid.draw(grob)

Update: this works with ggplot2 version 0.9.3 (although using grid is a fragile way to modify ggplot2 graphics)

grob[["grobs"]][["strip_t.1"]][["children"]][[2]][["label"]] <- expression(alpha[1])
grob[["grobs"]][["strip_t.2"]][["children"]][[2]][["label"]] <- expression(beta^2)
grob[["grobs"]][["strip_t.3"]][["children"]][[2]][["label"]] <- expression(hat(gamma))
grid.draw(grob)

Kuehnel

geditGrob has been removed from ggplot2: github.com/hadley/ggplot2/commit/… – Kuehnel

This example is failing for me with 0.9.3.1 with Error in getGrob(grob, "strip.text.x", grep = TRUE, global = TRUE) : It is only valid to get a child from a 'gTree'. – Halvaard

Answer 2

I tried it using the code below, and it works for me.

library("ggplot2")
library("grid")

## 1. Plot d and print d
d <- ggplot(iris, aes(Sepal.Length, Sepal.Width)) + geom_point() + facet_wrap(~Species)
d

## 2. Put Greek letters into facet labels using grid.edit function
grid.force()
grid.ls() #You should find the exact names for facet labels in the printed output!
grid.edit("GRID.text.77",label=expression(alpha[1])) #"GRID.text.77" = name for the first facet label
grid.edit("GRID.text.80",label=expression(beta^2)) #"GRID.text.80" = name for the second facet label
grid.edit("GRID.text.83",label=expression(hat(gamma))) #"GRID.text.83" = name for the third facet label

Warp