I found wp-codebox could highlight R code two years ago. This plugin is based on GeSHi to highlight source code internally.
Now there are many ways to highlight R syntax in the website. Pretty-R provided by Inside-R is a popular tool in the community.
I like the color style of Pretty-R more than which provided by GeSHi. GeSHi also links functions to the online manuals; this feature is very helpful for those not familiar with R. But I found many of the keywords are not linked properly.
I modified wp-codebox, to color the functions as in Pretty-R, and link the documents back to inside-R. The external link works fine and syntax now highlighted just exactly like the Pretty-R as you can refer to my previous post.
The modified file can be downloaded from github.