> Who decided √−1 belongs on a vertical axis?

It really doesn't. The imaginary axis is symmetrical and it makes sense to make it horizontal - there's no intrinsic physical difference between left and right. But it makes sense to map the real axis +ve/-ve onto (gravitational) up/down. That's why the Mandelbrot set looks so much more beautiful with that orientation (as it's displayed at https://fractal.institute/introduction-to-fractals/how-to-ge...) - it's a Buddha!