Class PriceLine

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.

Restyle in place; repaints. id is the hit-test handle the chart routes clicks and drags through, so it is not patchable — swapping it under a live drag would strand the gesture.

A last-price line is the case this exists for: it has to follow the tick direction, and only setPrice was updatable, so the colour was stuck at whatever it was constructed with.

Move the line; schedules a repaint via the host.

Layer order vs series: 'bottom' (behind), 'normal' (over), 'top' (overlay).