Add the option for the spot overlay to draw an extra layer.

Via a protected method called after the normal spots are drawn.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
Store min & max to ease drawing later.

Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
WIP: drawing the X-Y graph on the displayer.

Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>