r - 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?

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)