Stacked Bar Chart

I'm trying to create a a report that shows a 'Course Name' on X axis and the number of registration compared to the number of registrations need to make budget on the Y axis in a stacked bar. If a course needs to have 100 registrations to make budget and there are 80 registrations, I'd like the number of registrations number of registrations need to make budget to show 20. I can't seem to get it to work. Any ideas?