Fix category scatter issue where only the first filter-matching item was being labeled.
Fix category scatter issue where only the first filter-matching item
was being labeled.

Reduce dependence on hard-coded field/layer names to make more
amenable to subclassing.

Signed-off-by: Andy Boughton <5e68ec36f5178b44dbce03ea5a35443829b369bc@gmail.com>

statgen/locuszoom,statgen/locuszoom,statgen/locuszoom