Some plot layer types
have an associated shading parameter
which determines how plotted markers are coloured.
This is independent of the marker shapes
(which may be points, vectors, ellipses, ...)
but may be affected by how many markers are plotted on top of each other,
additional input table values, selected colour maps etc.
For the simplest shading types
(e.g. flat)
it's just a case of choosing a colour,
but the more complex ones have several associated parameters.
The various shading types and their usages are described in the following subsections.
auto
flat
translucent
transparent
density
aux
weighted
paux
pweighted