scale_second_axis: Scale secondary axis for background plotting
Description
Internal helper to rescale a secondary variable (s_axis) so it aligns
with the scale of a first variable (f_axis). Adds two new columns
to the data frame: s_axis_scale and s_axis_print.
Usage
scale_second_axis(background, df, dfby, f_axis, s_axis, by)
Value
The input data frame with two additional columns: