How do I shade the region between two curves in Grapher?
First select both equations in the sidebar, and then Equation » Integration… in the menu.



In the dialog, it has preselected Area Between Curves. Confirm it.

You can change the color in the inspector window.





Since your second equation is y=0, you could just select the first one, and integrate it, selecting Area in the dialog. The result should look the same.