Perhaps an easier solution - We have a right angled triangle, where opposite = 2 (radius), adj = 4 (it goes from 0,4 to 0,0). By observation the angle is 30deg
that means that the triangle completing the angle to 90 deg (to the horizontal) will have an angle of 60 - gradient is the tan of this triangle by observation, tan60 = sqrt3.
So by observation y = +/- sqrt3 +4.
Wordier than your solution, but if you can see it, it takes no working at all.
Not to say your solution isn't good as it is, I just wanted to offer an alternative. Wouldn't want to upset a female maths lover - after all we need more of you lot around