it's kind of the opposite to a sin graph
basically get the sin graph with pi/2 as the period, amplitude as 1
so from 0 to pi/4, the curve goes from 1 to 0 to 1, and then from pi/4 to pi/2, the curve starts from -1, goes to 0, and then goes to -1 again.
I'm crap at describing but hopefully you get the idea.