Take input data and define a mapping between faceting variables and ROW, COL and PANEL keys
combine_vars(data, env = emptyenv(), vars = NULL, drop = TRUE)
A list of data.frames, the first being the plot data and the subsequent individual layer data
The environment the vars should be evaluated in
A list of quoted symbols matching columns in data
should missing combinations/levels be dropped
A data.frame with columns for PANEL, ROW, COL, and faceting vars