Hadley Wickham has a couple of books which teach a lot about ggplot2. The first book I’d recommend is his and Garrett Grolemund’s R For Data Science book, which is available for free online:
To map an aesthetic to a variable, associate the name of the aesthetic to the name of the variable inside
aes(). ggplot2 will automatically assign a unique level of the aesthetic (here a unique color) to each unique value of the variable, a process known as scaling. ggplot2 will also add a legend that explains which levels correspond to which values.The colors reveal that many of the unusual points are two-seater cars. These cars don’t seem like hybrids, and are, in fact, sports cars! Sports cars have large engines like SUVs and pickup trucks, but small bodies like midsize and compact cars, which improves their gas mileage. In hindsight, these cars were unlikely to be hybrids since they have large engines.
Wickham also has the source to build his ggplot2 book online. If you don’t want to build the source, you also have the option of buying the book.