How to dynamically wrap facet label using ggplot2

Since this question was posted, the new label_wrap_gen() function with ggplot2 (>= 1.0.0, I think) handles this nicely:

facet_wrap(~groupwrap, labeller = labeller(groupwrap = label_wrap_gen(10)))

Note that you have to specify a width for it to work.

For older ggplot2 versions:

facet_wrap(~groupwrap, labeller = label_wrap_gen())

(too long as a comment, but not a real answer either)

I don't think a general solution will exist directly within ggplot2; it's the classic problem of self-reference for grid units: ggplot2 wants to calculate the viewport sizes on-the-fly, while the strwrap would need to know a firm width to decide how to split the text. (there was a very similar question, but I forget when and where).

You could however write a helping function to estimate how much wrapping you'll need before plotting. In pseudo code,

# takes the facetting variable and device size
estimate_wrap = function(f, size=8, fudge=1){ 

    n = nlevels(f)
    for (loop over the labels of strwidth wider than (full.size * fudge) / n){
     new_factor_level[ii] = strwrap(label[ii], available width)
    }

  return(new_factor)
}

(with some standard unit conversions required)

Of course, things would get more complicated if you wanted to use space="free".

Thanks to the guidance from @baptiste and @thunk, I created the function below, which seems to do a pretty good job of automatically wrapping facet labels. Suggestions for improvement are always welcome, though.

strwrap_strip_text = function(p, pad=0.05) { 
  # get facet font attributes
  th = theme_get()
  if (length(p$theme) > 0L)
    th = th + p$theme

  require("grid")
  grobs <- ggplotGrob(p)

  # wrap strip x text
  if ((class(p$facet)[1] == "grid" && !is.null(names(p$facet$cols))) ||
        class(p$facet)[1] == "wrap")
  {
    ps = calc_element("strip.text.x", th)[["size"]]
    family = calc_element("strip.text.x", th)[["family"]]
    face = calc_element("strip.text.x", th)[["face"]]

    if (class(p$facet)[1] == "wrap") {
      nm = names(p$facet$facets)
    } else {
      nm = names(p$facet$cols)
    }

    # get number of facet columns
    levs = levels(factor(p$data[[nm]]))
    npanels = length(levs)
    if (class(p$facet)[1] == "wrap") {
      cols = n2mfrow(npanels)[1]
    } else {
      cols = npanels
    }

    # get plot width
    sum = sum(sapply(grobs$width, function(x) convertWidth(x, "in")))
    panels_width = par("din")[1] - sum  # inches
    # determine strwrap width
    panel_width = panels_width / cols
    mx_ind = which.max(nchar(levs))
    char_width = strwidth(levs[mx_ind], units="inches", cex=ps / par("ps"), 
                          family=family, font=gpar(fontface=face)$font) / 
      nchar(levs[mx_ind])
    width = floor((panel_width - pad)/ char_width)  # characters

    # wrap facet text
    p$data[[nm]] = unlist(lapply(strwrap(p$data[[nm]], width=width, 
                                         simplify=FALSE), paste, collapse="\n"))
  }

  if (class(p$facet)[1] == "grid" && !is.null(names(p$facet$rows))) {  
    ps = calc_element("strip.text.y", th)[["size"]]
    family = calc_element("strip.text.y", th)[["family"]]
    face = calc_element("strip.text.y", th)[["face"]]

    nm = names(p$facet$rows)

    # get number of facet columns
    levs = levels(factor(p$data[[nm]]))
    rows = length(levs)

    # get plot height
    sum = sum(sapply(grobs$height, function(x) convertWidth(x, "in")))
    panels_height = par("din")[2] - sum  # inches
    # determine strwrap width
    panels_height = panels_height / rows
    mx_ind = which.max(nchar(levs))
    char_height = strwidth(levs[mx_ind], units="inches", cex=ps / par("ps"), 
                           family=family, font=gpar(fontface=face)$font) / 
      nchar(levs[mx_ind])
    width = floor((panels_height - pad)/ char_height)  # characters

    # wrap facet text
    p$data[[nm]] = unlist(lapply(strwrap(p$data[[nm]], width=width, 
                                         simplify=FALSE), paste, collapse="\n"))
  }

  invisible(p)
}

To use the function, call it in place of print.

library(ggplot2)
df = expand.grid(group=paste(c("Very Very Very Long Group Name "), 1:4),
                 group1=paste(c("Very Very Very Long Group Name "), 5:8),
                 x=rnorm(5), y=rnorm(5), stringsAsFactors=FALSE)

p = ggplot(df) +
  geom_point(aes(x=x, y=y)) +
  facet_grid(group1~group)
strwrap_strip_text(p)