The Seeing Theory website has some great tools for visualizing statistics, including interactive graphs, equations for determining various measurements, and explanations of concepts in statistics.

Fundamentals of Data Visualization is a free on-line book preview by Claus Wilke that provides how-to guides for creating graphs and infographics that best describe your data. Interesting side note: The book was written entirely in R Markdown, and the source code is on GitHub.

Need help remembering all those lines in RStudio? Check out these cheat sheets.

Biostatistical Design and Analysis Using R: A Practical Guide book. A free book on how to use R.