Stat* ObjectsAll stat_* functions (like stat_bin) return a layer that
contains a Stat* object (like StatBin). The Stat*
object is responsible for rendering the data in the plot.
Each of the Stat* objects is a ggproto object, descended
from the top-level Stat, and each implements various methods and
fields. To create a new type of Stat object, you typically will want to
implement one or more of the following: