Fill colour where the first series is above the second.
OptionalgradientSet to grade the band instead of flat-filling it, in place of
colorUp/colorDown. A point carrying its own color still overrides it.
Unset leaves the two-colour fill exactly as it was.
Optionalopacity0..1. Defaults to 0.12 — a band must not drown the candles it sits behind.
Fill colour where the second is above the first.