How to draw a color wheel in objective-c

I am trying to draw a color wheel for an iPhone, but I can't get the gradient to rotate around a point. I am trying to use gradients but objective-c provides a linear gradient, which draws a gradient in a straight line like this:

and a radial gradient, which draws the gradient starting at point and radiating out in all directions like this:

I want to draw a linear gradient that rotates around a point like this: