OptionalfallbackBars: () => readonly Bar[]Bars to position against where that series has none.
The second argument exists because a marker's series decides where it is drawn while the bar under it decides how high, and those are not always the same row of data. An indicator that draws one line in an uptrend and another in a downtrend has a gap in each, and a mark that lands in a gap had no bar to measure from and was dropped without a word. The caller passes the instrument's own bars, which have no gaps.
OptionalpriceScale: () => PriceScaleOptional: topmost hit under (x,y) in media px (relative to the pane plot).
The series whose pane and price scale the marks live on.