R: Data Visualization by Hadley Wickham

This is a 10x better ggplot2 tutorial than anything I've ever written. Granted, I wouldn't expect anything less.

The biggest strength of ggplot2 is the sheer amount of things you can do and get something presentable with only 2-3 lines of code. (something which can't be said about base R, despite a submission a couple months ago to the contrary)

Hadley Wickham is the creator of ggplot2. His online book R for Data Science has a chapter on visualization which, besides being a good tutorial on using ggplot2, happens to be the most concise explanation of the theory that informs its design (the grammar of graphics).