I just noticed there's no mention of the ":results graphics" header argument in 
the Org manual.

This worg documentation about source code blocks and R does mention it:

http://orgmode.org/worg/org-contrib/babel/languages/ob-doc-R.html

"If the value is :results graphics then 'base' graphics output is captured on 
disk, and a link to the graphics file is inserted into the Org Mode buffer (as 
is also the case with the graphics-only languages such as gnuplot, ditaa, dot, 
and asymptote.)"

(It goes on to say ":results output graphics" is necessary with ggplot2 images, 
but that's no longer true.)

I'm afraid I don't know enough about this header argument or its use elsewhere 
to offer a documentation patch, so I'm just pointing it out.

Working with R code blocks in Org is great.

Bill
-- 
William Denton ↔  Toronto, Canada ↔  https://www.miskatonic.org/