(This article was first published on Digithead's Lab Notebook, and kindly contributed to R-bloggers) To leave a comment for the author, please follow the link and comment on his blog: Digithead's Lab Notebook. R-bloggers.com offers daily e-mail updates about R news and tutorials on topics such as: visualization (ggplot2, Boxplots, maps, animation), programming (RStudio, Sweave, LaTeX, SQL, Eclipse, git,...