Interface ISeriesTransform

Optional: an independent copy of the state, for a live chart to push the forming bar through. push changes state on every call, so the bar that is still forming is pushed into a copy on each tick and into the transform itself only once it has closed. The built-in transforms all have one.

Optional: emit any in-progress element at end of data.

Feed one source bar; return 0..n newly completed derived elements.

Reset to initial state (start of a fresh batch / re-subscribe).