How do I print greek letters on the diagonal of a pairs plot in R?

Question

I want to create a pairs plot in R that has labels on the diagonal written as greek letters. I've tried creating a custom text.panel function that wraps the labels in an expression() call, but this does not work.

Here is a simple test case:

pairs.greek <- function(x, ...)
{
  panel.txt <- function(x, y, labels, cex, font, ...)
  {
    lab <- labels
    text(0.5, 0.5, expression(lab), cex=cex, font=font)
  }
  pairs(x, text.panel=panel.txt)
}
dat <- data.frame(alpha=runif(10), beta=runif(10), gamma=runif(10))
pairs.greek(dat)

Answer 1

expression(lab) doesn't actually evaluate lab so you end up with all the labels being lab. Instead, you could change that line to:

text(0.5, 0.5, parse(text=lab), cex=cex, font=font)

which will do what you want. Note that the pairs function also accepts a labels argument, so this would work too:

pairs(dat, labels=c(expression(alpha), expression(beta), expression(gamma)))