Boxplots (2-way)

This is a very useful chart to start with when you have a collection of multivariate data and are trying to make sense of it.

The chart uses the same spacing conventions as a multiple barchart; here I have set the gap between groups to be 3 bar widths and put the tickmarks between the groups. This is "real" data but can you spot the rather obvious error? It went un-noticed by several eminent authors of classic statistical texts!