That’s still just convention. In cartesian systems the only requirement is that x and y are orthogonal.
When you add a third dimension, and additional concern arises being what direction of z is the positive one. You’ll notice the two examples are not the same, if you flip either one so the x/y line up, there is still disagreement about which direction z is pointing.
In a 2 dimensional graph y is vertical, adding z doesn’t change that.
That’s still just convention. In cartesian systems the only requirement is that x and y are orthogonal.
When you add a third dimension, and additional concern arises being what direction of z is the positive one. You’ll notice the two examples are not the same, if you flip either one so the x/y line up, there is still disagreement about which direction z is pointing.