Change denominator in Chart View for a Folder

I have been asked to fix the chart view for a folder. They want to see the percentage of completed tasks by department.  The current chart view is showing the percentage of the number of tasks per department / total number of tasks in the folder.  We have a custom field by department that is being using to break up the chart but not in the way we want. We want it to show the number of completed per department / total number of tasks per department. For some reason, no matter how I filter it won't give me the denominator I want. For example, I can get it to give me the total completed per department in the numerator but the denominator is the total completed for the entire folder. The only solution I can think of is separate folders by department.

Any suggestions would be amazing! Thank you!