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 ggplot

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).