x, y and dots.aes_group() uses a list of x's and y's to create each
unique combination with dots.
aes_group(lst)aes_group() will be
the list produced by aes_assing().aes_group() uses an lapply loop to give every dots
element with a copy of the x and y vectors (if any). This
creates a list in which the first set of components correspond to the
combination of dots elements, and the second set of components (the
nested components) correspond to the x and y vectors.