r - How do I overlay an image on to a ggplot?

Question

I'd like to read an image from the web. e.g.

http://api.altmetric.com/donut/502878_64x64.png

and insert it into the top right of a ggplot

df <- data.frame(x=1:10, y=sample(1:100,10))
# a fake plot to try it on.
ggplot(df, aes(x,y)) + geom_point(size = 2)

How would I do this?

Answer 1

You are looking for annotation_raster and readPNG

mypngfile <- download.file('http://api.altmetric.com/donut/502878_64x64.png', destfile = 'mypng.png', mode = 'wb')
library(png)
mypng <- readPNG('mypng.png')


p <- qplot(mpg, wt, data = mtcars) + theme_bw()
p + annotation_raster(mypng, ymin = 4.5,ymax= 5,xmin = 30,xmax = 35) + 
    geom_point()

These new features (and more examples) are described here