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).
A single live reading after the params (typically crosshair-driven).
Optional
One reading per plot, each in its own color.
Layer order vs series: 'bottom' (behind), 'normal' (over), 'top' (overlay).
Optional: expand the pane's autoscale range so this primitive isn't clipped.