Data Analysis with R, Second Edition - Second Edition

Categorical and continuous variables

We have seen that box plots are a great way of comparing the distribution of a continuous variable across different categories. As you might expect, box plots are very easy to produce using ggplot2.

The following snippet produces the box-and-whisker plot that we saw earlier, depicting the relationship between the petal lengths of the different iris species in the iris dataset:

 library(ggplot) 
 qplot(Species, Petal.Length, data=iris, geom="boxplot",  
         fill=Species) 

First, we specify the variable on the x axis (the iris species) and then the continuous variable on the y axis (the petal length). Finally, we specify that we are using the iris dataset, that we want a box plot, and that we want to fill the boxes ...