Dodging in ggplot2

Zenguo Zhang teaches a lesson:

In ggplot2, when displaying grouped data along categorical axes (such as grouped bar charts, boxplots, or error bars), horizontal dodging is used to prevent elements from overlapping at each categorical position. ggplot2 provides two primary dodging position functions: position_dodge() and position_dodge2().

Click through or else Piccolo is going to yell again.