SSAS/SSRS : Calulating sums and averages with M2M dimensions

Today a post about calculating averages of departments and these measures are compared to a specific department. I have a requirement to build a report with a graph with values for a specific department. Another demand is to build a line in the graph with averages of all (?) of the departments of the organization. The report must allow the user to select the department from a dropdown list in a report parameter. Based on the selected department the report should show measures for the selected department and an average of all departments.