Mathematical symbols in ggplot2 0.9.2

Question

If you want to add mathematical symbols to facet_wrap , you used to use the grid package, as in (an example is taken 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 does not work anymore 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 to add math symbols in version 0.9.2?

Answer 1

Refresh . It may be easier to use the facet_wrap_labeller function 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 

The original solution with a little editing to make it easier to select the text strip.

It works, but in the middle it gets a little dirty. It uses ggplot2 0.9.2 and functions from the grid and gtable . There should be an easier way, and I hope you can do something to develop a simpler solution.

Steps:

• Draw the original plot;
• Find the stripe item you want to change;
• Remove this strip element;
• Change "D" to expression(Y[1]) ;

• Insert the modified stripe element back into the original graph.

   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 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 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) 

Result: