Dynamic Pie Chart legend based on hierarchy selection

I have a hierarchy for a company (i.e. Department -> Section -> Workgroup) with 2 drop down lists on the page - one for Department and one for Section.  When a user selects a Department the Sections drop down list is automatically filtered down to the relevant Sections within the Department.  

I'd like to change a Pie chart's legend / grouping based on what the user selects in the Department and Section drop down lists.  

• If no Departments are selected, then the Pie chart would show a breakdown of Items per Department (i.e. All Departments across the company)  
• If a Department is selected but no Section is selected then the Pie chart would show a breakdown of Items per Section for the selected Department (i.e. All Sections for the relevant Department) 
• If a Department and Section are selected then the Pie chart would show a breakdown of Items per Workgroup for the Section selected (i.e. All Workgroups for the relevant Section and Department)

Is this possible?  I've been researching various reporting model pattens but can only find dynamic axis based on a single slicer of groups and not a hierarchy slicer selection.

Thanks in advance for any help you can give me to point me in the right direction.