Replace categorical x-axis labels with images

add_images_as_xlabels.R

#' Replace categorical x-axis labels with images
#'
#' Pipe a ggplot2 graph (with categorical x-axis) into this function with the argument of a list of 
#' pictures (e.g. loaded via readImage) and it builds a new grob with the x-axis categories 
#' now labelled by the images. Solves a problem that you perhaps shouldn't have.
#'
#' @author J. Carroll, \email{jono@@jcarroll.com.au}
#' @references \url{http://stackoverflow.com/questions/29939447/icons-as-x-axis-labels-in-r-ggplot2}
#' 
#' @param g     ggplot graph with categorical x axis
#' @param pics  ordered list of pictures to place along x-axis
#'
#' @return NULL (called for the side-effect of producing a new grob with images for x-axis labels)
#' 
#' @import grid 
#' @import ggplot2
#' 
#' @export
#'
#' @example 
#' \dontrun{ggplot(data, aes(x=factor(x),y=y)) + geom_point() %>% add_images_as_xlabels(pics)}
#' 
add_images_as_xlabels <- function(g, pics) {

  ## ensure that the input is a ggplot
  if(!inherits(g, "ggplot")) stop("Requires a valid ggplot to attach images to.")
    
  ## extract the components of the ggplot
  gb   <- ggplot_build(gg)
  xpos <- gb$panel$ranges[[1]]$x.major
  yrng <- gb$panel$ranges[[1]]$y.range
  
  ## ensure that the number of pictures to use for labels 
  ## matches the number of x categories
  if(length(xpos) != length(pics)) stop("Detected a different number of pictures to x categories")
  
  ## create a new grob of the images aligned to the x-axis
  ## at the categorical x positions
  my_g <- do.call("grobTree", Map(rasterGrob, pics, x=xpos, y=0))
  
  ## annotate the original ggplot with the new grob
  gg <- gg + annotation_custom(my_g,
                               xmin = -Inf, 
                               xmax =  Inf,
                               ymax = yrng[1] + 0.25*(yrng[2]-yrng[1])/npoints, 
                               ymin = yrng[1] - 0.50*(yrng[2]-yrng[1])/npoints)
  
  ## turn off clipping to allow plotting outside of the plot area
  gg2 <- ggplotGrob(gg)
  gg2$layout$clip[gg2$layout$name=="panel"] <- "off"
  
  ## produce the final, combined grob
  grid.newpage()
  grid.draw(gg2)
  
  return(invisible(NULL))
  
}

@baptiste you're 100% correct and your method is vastly superior/exactly what I was after. New gist: https://gist.github.com/jonocarroll/2f9490f1f5e7c82ef8b791a4b91fc9ca and blog post: http://jcarroll.com.au/2016/06/03/images-as-x-axis-labels-updated/ (will be scraped to R-bloggers sometime today). Thank you very kindly for your help.

Any chance of a new push to get this into ggplot2 officially?

 gb   <- ggplot_build(gg)

that's a typo, should be ggplot_build(g)