I was actually thinking about the following. Could it make sense to improve the sorter helper (maybe with a better name to subsume most of the various helpers we use) so that the pair syntax in the palette is not needed?
Essentially, one could use :c => sorter("y", "x", "z") to establish levels: "y" is 1, "x" is 2 and "z" is 3, regardless of what data is actually present. That way, passing the same palette will always yield consistent plots (regardless of data filtering), but now the palette is strictly a “theme component” (not data specific).