[Off-topic] beautiful graph :O

Thanks, that is a really interesting tesselation of squares.  

I never would have guessed that sinx+cosx = 0 would look like that.

Mmm   and WHY is it so ??

Let's see:

your graph is in radians, I have done another in degrees because I think more people will understand.

https://www.desmos.com/calculator/9dj7jlgab6

sinx=-cosy

This will be in the 2nd and third quadrant.

\(sinx=-sin(180\pm x) \qquad in\;degrees\\ y=180\pm x+360n\\ y=x+180 \qquad and \qquad y=-x+180\\ y=x+540 \qquad and \qquad y=-x+540\\ y=x+900 \qquad and \qquad y=-x+ 900\\ etc \)

This is how you end up with that criss cross (tesselating squares pattern)

By the way - I do not consider this off topic at all !!