I think by default it plots from -10 to 10 on both axis. So if you plot y=x and y=-x together they will not look perpendicular on the rectangular screen. You ca

I think by default it plots from -10 to 10 on both axis. So if you plot y=x and y=-x together they will not look perpendicular on the rectangular screen. You can of course adjust the axes, but if I remember right it's not obvious how to make them match the screen. Graphing tools on a computer or smartphone probably have better default settings.