It's conventional.
Letters at the front of fthe alphabet are used as constants, those at the end as variables.
So, s to represent distance travelled, t to represent time, u and v to represent velocity, x, y and z as labels for the (three dimensional) co-ordinate axes, w tends not to be used for some reason.
When dropping back to two dimensions, z is the one not used, so for graphical work we tend to use x and y.
It's conventional.
Letters at the front of fthe alphabet are used as constants, those at the end as variables.
So, s to represent distance travelled, t to represent time, u and v to represent velocity, x, y and z as labels for the (three dimensional) co-ordinate axes, w tends not to be used for some reason.
When dropping back to two dimensions, z is the one not used, so for graphical work we tend to use x and y.
