dot-prep_aes.Rdggplot2 geoms have default aesthetics which can be changed for each
session. This function is a generalised way to set geom defaults. It only
changes settings where the default is not NA; no need for geom_point
to have a default font family.
.prep_aes(geom, aes, setting)(character) the geom to change (e.g. point or
line)
(character) the aesthetic to change (e.g. colour or
alpha)
(various) what to set the aesthetic (e.g. a colour or a number)
the default aesthetic ready to be set
This is used inside `.set_grattab_aesthetics()`