In order to have a brief data introduction, step 1 starts by calling ?cars. This is a very useful way to get to meet variables and background related to almost every data set coming from a package. Once ggplot2 requires data coming from data frames, class() function is checking if is that the case, answer is affirmative. At the end of this step head() function is checking upon the first six observations.
Moving on to step 2, after loading ggplot2, it demonstrates how to store the basic coordinate mapping and aesthetics into an object called plot1 (try it on the class() function). In order to set the basics, it uses a function (ggplot()) that initializes every single ggplot.
Storing a plot coming from ggplot2, ggvis, or plotly ...
