math symbols in ggplot2 0.9.2

Question

When you want to add math symbols to facet_wrap you used to be able to use the grid packge as in (example taken from here):

library(ggplot2); library(grid)
d <- ggplot(diamonds, aes(carat, price, fill = ..density..)) 
    xlim(0, 2) + stat_binhex(na.rm = TRUE) + opts(aspect.ratio = 1)
d <- d + facet_wrap(~ color, ncol = 4)
grob <- ggplotGrob(d)
strip_elem <- grid.ls(getGrob(grob, "strip.text.x", grep=TRUE, global=TRUE))$name
grob <- editGrob(grob, strip_elem[1], label=expression(Y[1]))
grid.draw(grob)

This no longer works as it gives this error:

>  strip_elem <- grid.ls(getGrob(grob, "strip.text.x", grep=TRUE, global=TRUE))$name
Error in getGrob(grob, "strip.text.x", grep = TRUE, global = TRUE) : 
  It is only valid to get a child from a 'gTree'
> grob <- editGrob(grob, strip_elem[1], label=expression(Y[1]))
Error in editGrob(grob, strip_elem[1], label = expression(Y[1])) : 
  object 'strip_elem' not found

How can we add math symbols in version 0.9.2?

Answer 1

Update It might be easier to use a facet_wrap_labeller function available from here.

library(ggplot2)
library(gtable)

d <- ggplot(diamonds, aes(carat, price, fill = ..density..)) +
    xlim(0, 2) + stat_binhex(na.rm = TRUE) + theme(aspect.ratio = 1)
(d <- d + facet_wrap(~color, ncol = 4))    

facet_wrap_labeller <- function(gg.plot, labels=NULL) {
  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)) 
  return(g)
}

# Some arbitrary strip texts
StripTexts = expression(Y[1], E, F, G, H, I, J)

# Apply the facet_wrap_labeller function
g = facet_wrap_labeller(d, StripTexts)

# Draw it
g

Original solution with a minor edit to make selecting the strip text a little easier.

This works but it gets a bit messy in the middle. It uses ggplot2 0.9.2 and functions from the grid and gtable packages. There has to be a simpler way, and I'm hoping you can take something from this to work up a simpler solution.

The steps are:

• Draw the original plot;
• Locate the strip element that needs to be changed;
• Extract that strip element;
• Change "D" to expression(Y[1]);

• Insert the modified strip element back into the original plot.

  library(ggplot2); library(grid); library(gtable)
  d <- ggplot(diamonds, aes(carat, price, fill = ..density..)) +
      xlim(0, 2) + stat_binhex(na.rm = TRUE) + theme(aspect.ratio = 1)
  (d <- d + facet_wrap(~color, ncol = 4))    
  # The aim is to change the strip text "D" to expression(Y[1])
  
  # Winston's accessor function:
  gt_getgrob <- function(gt, pattern) {
    idx <- grep(pattern, gt$layout$name)
    if (length(idx) > 1)
      stop("More than one match for pattern '", pattern, "'")
    gt$grobs[[idx]]
  }
  
  g = ggplotGrob(d)
  g$layout    # We want "strip_t-1"
  strip <- gt_getgrob(g, "t-1")        # Use the accessor function to extract it.
  str(strip)     # Locate the "D" label
  
  strip$children[[2]]$label = expression(Y[1])    # Change the label.
  grid.draw(strip)       # Yes, it's worked
  
  gt <- ggplot_gtable(ggplot_build(d))  
  gtable_show_layout(gt)       # Get the layout of the original plot.
  
  gt = gtable_add_grob(gt, strip, t=3, l=4, b=3, r=4)  # Insert the modified strip element into the plot.
  grid.newpage()
  grid.draw(gt)  
  

The result is: