Layer: one geom + stat + data/mapping overrides
A Geom subclass instance.
A Stat subclass instance.
Layer-level aesthetic mapping (or NULL to inherit).
Layer-level data (or NULL to inherit the plot data).
Named list of literal visual parameters (e.g. size = 4).
Whether the layer merges the plot-level mapping into its
own (TRUE for data layers; reference-line layers that synthesize
their own data set FALSE).
An S7 object of class Layer, as returned by the geom_*() constructors.