Stacked Bar Graph With Average Line

We are trying to create a stacked bar graph with an average line. Each time we try to add in the average data to the graph it adds it to the stacked bar. The stacked bar should represent the inpatient & outpatient cases at a hospital for the current year (months showing across the as columns). The line should represent the previous 2-yr average total. Anyone have an idea how to add a line to the stacked bar graph? It has us stumped.