// helper for returning the weekends in a period
function weekendAreas(axes) {
	var markings = [];
	var d = new Date(axes.xaxis.min);
	// go to the first Saturday
	d.setUTCDate(d.getUTCDate() - ((d.getUTCDay() + 1) % 7))
	d.setUTCSeconds(0);
	d.setUTCMinutes(0);
	d.setUTCHours(0);
	var i = d.getTime();
	do {
		// when we don't set yaxis the rectangle automatically
		// extends to infinity upwards and downwards
		markings.push({ xaxis: { from: i, to: i + 2 * 24 * 60 * 60 * 1000 } });
		i += 7 * 24 * 60 * 60 * 1000;
	} while (i < axes.xaxis.max);

	return markings;
}