returns the first value of expression sorted by corresponding sort-weight when expression is iterated over the chart dimension(s). Sort-weight should return a numeric value where the lowest value will render the corresponding value of expression to be sorted first. By preceding the sort-value expression with a minus sign, the function will return the last value instead. If more than one value of expression share the same lowest sort-order, the function will return null. By stating an n larger than 1, you will get the nth value in order.