Changing facet label to math formula in ggplot2

You can edit the grobs in the gtable,

ggplot(diamonds, aes(carat, price, fill = ..density..)) +
  xlim(0, 2) + stat_binhex(na.rm = TRUE) + facet_wrap(~ color, ncol = 4)


for(ii in 1:7)
grid.gedit(gPath(paste0("strip_t-", ii), "strip.text"), 
           grep=TRUE, label=bquote(gamma[.(ii)]))

alternatively, if you want to save a grob,

g <- ggplotGrob(d)
gg <- g$grobs

strips <- grep("strip_t", names(gg))
for(ii in strips)
  gg[[ii]] <- editGrob(getGrob(gg[[ii]], "strip.text", 
                               grep=TRUE, global=TRUE), 
                       label=bquote(gamma[.(ii)]))

g$grobs <- gg

using ggsave would require extra (ugly) work, since one has to fool the test for class ggplot… I reckon it will be easier to call pdf() ; grid.draw(g); dev.off() explicitly.

---

Edit by Roland:

I made a small correction and wrapped it in a function:

facet_wrap_labeller <- function(gg.plot,labels=NULL) {
  #works with R 3.0.1 and ggplot2 0.9.3.1
  require(gridExtra)
  
  g <- ggplotGrob(gg.plot)
  gg <- g$grobs      
  strips <- grep("strip_t", names(gg))
    
  for(ii in seq_along(labels))  {
    modgrob <- getGrob(gg[[strips[ii]]], "strip.text", 
                       grep=TRUE, global=TRUE)
    gg[[strips[ii]]]$children[[modgrob$name]] <- editGrob(modgrob,label=labels[ii])
  }
  
  g$grobs <- gg
  class(g) = c("arrange", "ggplot",class(g)) 
  g
}

This allows to print nicely and even ggsave can be used.