How to shade region between polar curves in Grapher?

I've graphed two polar curves and would like to shade the region inside one and outside the other. When I select the two equations and then select Integrate from the Equation menu, I get the error message > Cannot integrate the difference between the selected graphs. It seems simple. Why can't Grapher do it? In case it makes a difference, the two equations are r = 6 (a circle) and r = 6 - 6 sin(theta) (a cardioid). I want to shade the region inside the cardioid but outside the circle.