A tiny arithmetic expression evaluator for computed aesthetics — so
aes can map e.g. "pop / 1e6", "log(gdp)", or "a * b + 1" instead of a
bare column name. Anything that isn’t an existing column is parsed and
evaluated per row against the data’s numeric columns.
A post-scale aesthetic derivation (R’s after_scale): the target color
aesthetic is set to the source aesthetic’s mapped color, adjusted in
lightness (+ toward white, - toward black, in -1.0..=1.0).