9.5

• Once you map an aesthetic, ggplot2 takes care of the rest by
  • selecting a reasonable scale to use with the aesthetic,
  • and it constructing a legend that explains the mapping between levels and values.
• For x and y aesthetics, ggplot2 does not create a legend, but it creates an axis line with tick marks and a label.
  • The axis line acts as a legend; it explains the mapping between locations and values.