Optional: expand the pane's autoscale range so this primitive isn't clipped.
Optional: topmost hit under (x,y) in media px (relative to the pane plot).
Show a dimmed reference line at the pre-drag price while the user drags (pass the original price on drag start, null on drag end to clear).
Update the info segment text (e.g. live position P&L); repaints.
Move the line; schedules a repaint via the host.
Layer order vs series: 'bottom' (behind), 'normal' (over), 'top' (overlay).
Optional: expand the pane's autoscale range so this primitive isn't clipped.