Scatterplots are probably among the most common plots, since they are frequently used to display the relationship between two quantitative variables. When two variables are provided, ggplot2 will make a scatterplot by default. Now that you have already acquired some experience from the previous sections of this chapter, the representation of the scatter plot will be quite straightforward for you.

For our example on how to build a scatterplot, we will use a dataset called ToothGrowth, which is available in the base R installation. Reported in this dataset are measurements of the length of the teeth of 10 guinea pigs for three different doses of vitamin C (0.5, 1, and 2 mg). It is delivered in two different ways—as orange juice or as ascorbic acid (a compound with vitamin C activity). You can find details on the dataset help page at ?ToothGrowth.

We are interested in seeing how the length of the teeth changed for each different dose. We are not able to distinguish the different...