Builds a fan chart's points: one per median position, carrying each band's
low edge from the outermost in, the median at 0.5, then each high edge from
the innermost out -- the order PercentileBandTrace reads its rows in. A
band that draws nothing at a position is a gap there.
Builds a fan chart's points: one per median position, carrying each band's low edge from the outermost in, the median at 0.5, then each high edge from the innermost out -- the order
PercentileBandTracereads its rows in. A band that draws nothing at a position is a gap there.