A boxplot summarizes the distribution of a continuous variable. It displays its median, its first and third quartiles and its outliers. Main caveat is that the underlying distribution is hidden. This page explains how to build a basic boxplot with ggplot2.
The ggplot2 library allows to make a boxplot using geom_boxplot(). You have to specify a quantitative variable for the Y axis, and a qualitative variable for the X axis ( a group).
This document is a work by Yan Holtz. Any feedback is highly encouraged. You can fill an issue on Github, drop me a message on Twitter, or send an email pasting yan.holtz.data with gmail.com.