If "year" field is numeric you can do something like (without cuotes):
If you put the expresion "=year(MAX(end_date))" in a variable you can simplify your expresion in this way:
SET vMaxYear = '=year(MAX(end_date))';
Variable vMaxYear is dynamic relatively of the selections, but is not afected by the dimension of the graphic when you put in a expresion.
What is the dimension of your graphic? You should keep in mind that if Percent is a field precalculated in Load Script time as a Monthly percent when is aggregated by Months in a Qlikview graphic and totalize, monthly value shows fine but the total not.