Annotate ggplot2 facets with images

Question

Say I'm graphing data for 4 people: Alice, Bob, Chuck and Dana. I'm using ggplot2 to make a faceted plot with one facet per person. I also have 4 images on disk: Alice.png, Bob.png, Chuck.png and Dana.png. (obviously this is a synthetic example that would need to scale to more than 4 facets :)

Is there a way I can annotate each facet with the corresponding image, ideally instead of the facet label (although I'd be happy with the image right below the label)? Perhaps something similar to the technique used here: Use image instead of labels in ggplot2 legend ? I've tried reading through the documentation for the various annotate methods but my R-fu is insufficient to the challenge!

Could you post a reproducible example of your faceted plot, to get people started? — Marius

baptiste baptiste · Accepted Answer · 2012-10-31T06:31:43

Not very elegant, but you can add grobs on top of the strip labels,

library(ggplot2)

d <- expand.grid(x=1:2,y=1:2, f=letters[1:2])
p <- qplot(x,y,data=d) + facet_wrap(~f)

g <- ggplot_gtable(ggplot_build(p))

library(gtable)
library(RCurl)
library(png)
shark <- readPNG(getURLContent("http://i.imgur.com/EOc2V.png"))
tiger <- readPNG(getURLContent("http://i.imgur.com/zjIh5.png"))

strips <- grep("strip", g$layout$name)
new_grobs <- list(rasterGrob(shark, width=1, height=1),
                  rasterGrob(tiger, width=1, height=1))
g <- with(g$layout[strips,],
          gtable_add_grob(g, new_grobs,
                          t=t, l=l, b=b, r=r, name="strip_predator") )        
grid.draw(g)

Edit: you can also replace directly the grobs,

strips <- grep("strip", names(g$grobs))
new_grobs <- list(rectGrob(gp=gpar(fill="red", alpha=0.2)),
                  rectGrob(gp=gpar(fill="blue", alpha=0.2)))
g$grobs[strips] <- new_grobs
grid.draw(g)

Annotate ggplot2 facets with images

1 Answers