Greek letters in ggplot strip text

Question

I'm trying to override the text in some ggplot strips to incorporate Greek characters. Here's some sample data, and the base for the plot.

dfr <- data.frame(
   x = rep(1:10, times = 6),
   y = runif(60),
   fx = rep(c("foo", "bar"), each = 30),
   fy = rep(c("alpha", "beta", "gamma"), each = 10, times = 2)
)

p <- ggplot(dfr, aes(x, y)) + geom_point()

My first attempt at a plot has no Greek in the strip labels.

 p + facet_grid(fy ~ fx)

I gather that I'm supposed to add a labeller argument to facet_grid to override the text. I presumed that this should spit out an expression to handle the greek characters, but my code just throws an error when the graphic is printed.

lbl <- function(variable, value)
{
   if(variable == "fy") parse(text=as.character(value)) else value
}
p + facet_grid(fy ~ fx, labeller = lbl)


Error in aperm(X, c(s.call, s.ans)) : 
  unimplemented type 'expression' in 'aperm'

How should I be creating the strip labels?

score 10 · Accepted Answer · answered Mar 08 '10 at 18:24

10

Try this:

p + facet_grid(fy ~ fx, labeller = label_parsed)

answered Mar 08 '10 at 18:24

Leo Alekseyev

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I see that this answer works, but I can't figure out how. Where does label_parsed come from? I can't find it in the online ggplot docs. – kmm Mar 10 '10 at 19:30
2

It is mentioned briefly in the package docs (http://cran.r-project.org/web/packages/ggplot2/index.html); I agree that it could be better documented -- I only knew it b/c I've seen Hadley mention it at some point. – Leo Alekseyev Mar 10 '10 at 20:56
Thanks -- I see it now. That's a potentially very useful command. – kmm Mar 11 '10 at 04:03

score 4 · Answer 2 · edited Jan 19 '12 at 16:38

posting this here since it's related:

If you want the name of the variable itself as well as the levels/values of the variable to be evaluated as an expression (i.e. rendered as if they were latex), try this:

label_parseall <- function(variable, value) {
    plyr::llply(value, function(x) parse(text = paste(variable, 
        x, sep = "==")))
}

Example:

data <- data.frame(x = runif(10), y = runif(10), 
    gamma = sample(c("gamma[0]", "gamma[1]"), 10, rep = T))
ggplot(data, aes(x, y)) + geom_point() + facet_grid(~gamma, 
    labeller = label_parselabel)

enter image description here

image at http://img709.imageshack.us/img709/1168/parseall.png

Nice! This should be default behaviour. – Kahovius May 17 '22 at 15:10 — Kahovius, May 17 '22 at 15:10

Greek letters in ggplot strip text

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