Code to make your coordinate cartesian, the 0,0 is in the middle of the view

gistfile1.m

CGContextRef ctx = UIGraphicsGetCurrentContext();
CGSize sz  = [self bounds].size;
	
CGContextTranslateCTM(ctx, 0.0f, sz.height);
CGContextScaleCTM(ctx, 1.0f, -1.0f);
CGContextTranslateCTM(ctx, sz.width * 0.5f, sz.height * 0.5f);