Hi Rodrigo,
That piston thing was indeed my inspiration.
I've modified the CodePen so it looks like what I tried to achieve, but it's a cheat solution.
It looks like I'm drawing the green arch while rotating the redRadius, but it has been there all the time.
I'm just hiding it behind a rectangle, which I push outwards with the convertCoordinates method.
Right now I don't think it's possible to control the endpoint of an arch with the convertCoordinates method.