Note: this sets colour, not fill — fill is the value it reads
from, colour is what it derives. Maps colour to a blended version of
fill, evaluated after scale training via ggplot2::after_scale().
Attaches no scale itself — use scale_fill_colourblend_discrete() (or
_continuous/_binned) for a ready-to-plot version that also scales
fill. Requires the ggrefine function to be used instead of ggplot2::ggplot to
work where no fill aesthetic is mapped.
Value
A ggplot2 mapping object (as returned by ggplot2::aes()).