// asAtDate must be a string of type yyyymmdd
	// function returns price, or -1 if out of range
	function getHistoricalPrice(asAtDate)
	{
		var count = wlOSHDateArr.length;
		var date = parseInt(asAtDate);
		if (date > wlOSHDateArr[count-1] || date < wlOSHDateArr[0])
			return -1;

		var lower = 0;
		var upper = count-1;
		var index = parseInt(upper/2);
		var curDate;

		// binary search for date
		while(upper - lower > 1)
		{
			curDate = wlOSHDateArr[index];
			if (curDate == date)
				return wlOSHPriceArr[index];
			if (curDate > date)
				upper = index;
			else
				lower = index;
			index = parseInt((upper+lower)/2);
		}
		// Make sure that what we've found is actually the right day.
		if (wlOSHDateArr[upper] == date) {
			return wlOSHPriceArr[upper];
		} else {
			return -1;
		}
	}