Which Wave is leading?
There is more than one right answer to this question. The purple wave could be leading the blue one by 135˚ or it could be lagging behind by 225˚. The blue curve is just a standard sine wave, y = sin(x), and the purple curve could be described as either y = sin(x - 135) or y = sin(x + 225) because the two produce the same curve, due to the fact that sine has a period of 360˚.