underlayCallback: function(canvas, area, g) {
var bottom_left = g.toDomCoords(highlight_start, -20);
var top_right = g.toDomCoords(highlight_end, +20);
underlayCallback: function(canvas, area, g) {
var bottom_left = g.toDomCoords(highlight_start, -20);
var top_right = g.toDomCoords(highlight_end, +20);