R/facet_wrap_labeller.R

In johnDorian/ggsnippets: A few addons for the ggplot2 package

#' A function to label facetted ggplots
#' 
#' This function alters the lables of a facetted ggplot object. This is useful if you want
#' use expressions or have a simple way to mdify the labels of a facetted ggplot object.
#'
#' @title A function to allow for expressions in ggplot facet titles. 
#' @description Allows for the use of expressions with facet titles.
#' @param gg.plot a ggplot object
#' @param labels labels for each facet.
#' @return a ggplot object with new facet titles. 
#' @references \url{http://stackoverflow.com/questions/11979017}
#' @seealso plotmath
#' @export
#' @examples 
#' ## Add a factor to the cars example data to facet the plots
#' library(ggplot2)
#' x_cars <- data.frame(cars, variable = sample(letters[1:5],
#'                                              50,
#'                                              replace = TRUE))
#' ## Create the plot with facets
#' plt <- ggplot() + geom_point(aes(speed, dist), x_cars) + 
#'        facet_wrap(~variable, ncol=1)
#' plt
#' ## An example without labels - no real point in this. 
#' library(gridExtra)
#' facet_wrap_labeller(plt)
#' ## Adding expressions within the facet labels. 
#' facet_wrap_labeller(plt, labels = c(expression(a^2), 
#'                                    expression(b[1]), 
#'                                    expression(frac(c,2)),
#'                                     "d",
#'                                      expression(infinity)))
#'  
#'                                      

 
facet_wrap_labeller <- function(gg.plot,labels=NULL) {
  if(!inherits(gg.plot, "ggplot")){
    stop( "gg.plot must be a ggplot object (see ?ggplot)")
  }
  g <- ggplot2::ggplotGrob(gg.plot)
  gg <- g$grobs      
  strips <- grep("strip_t", names(gg)) #Get the names - should be same length as labels. 
  if(length(strips)==0){
    stop("Your ggplot does not seem to have any facets.")
  }
  for(ii in seq_along(labels))  {
    modgrob <- grid::getGrob(gg[[strips[ii]]], "strip.text", 
                       grep=TRUE, global=TRUE)
    gg[[strips[ii]]]$children[[modgrob$name]] <- grid::editGrob(modgrob,label=labels[ii])
  }
  
  g$grobs <- gg
  class(g) = c("arrange", "ggplot",class(g)) 
  g
}