Class Footprint

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).

Called once for each zone that showZones extends (so nothing is reported while it is off) when a bar's range comes back to it after it became active (since 2.6.4), the moment to raise an alert or play a sound. The first bars a footprint is given are history and report nothing; each later setBars is read as the live tail, so a touch in the bar that was newest at the previous call, or in any bar after it, is reported. setBars([]), or bars that start again from an older bar, begin a new history. Returns the unsubscribe.

Multiple nodes, big orders, absorption, flips, divergence, zones and unfinished auctions for the loaded bars, under the current options. Zones carry activeTime and endTime, so a host can raise its own alerts.

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