See also

• ggplot2 Cheatsheet made by Rstudio can be found at  https://www.rstudio.com/wp-content/uploads/2015/03/ggplot2-cheatsheet.pdf

• Learn layers from ggplot2's author and a real R-universe star at http://rpubs.com/hadley/ggplot2-layers

• Did you know that gg's + is actually a shortcut for a function? A clue on that and some exercises are hidden at http://rpubs.com/hadley/97970 

• Learn more about ggvis layers and how they can be translated into ggplot2 ones at http://ggvis.rstudio.com/layers.html

• Learn more about ggvis scaled and unscaled arguments at http://ggvis.rstudio.com/properties-scales.html