ggplot2 style and stand-alone aesthetic arguments (i.e.
y, x:z, etc).mpg) or dplyr-like variable calls (i.e.
mpg:hp).
rm.gg2(expr)substitute(). If the tree is not
wrapped by c() then it is advised to wrap x with
list().x in a list is due to the way
x will be indexed/subsetted. The c function wrapping is
assumed, so therefore the list wrapping is needed.