If I say that $J$ (the volume current density) is proportional to $s$, the distance from the center axis of a long cylindrical wire, I can say that as $s$ changes, so does $J$.

I feel as though I could say the same thing as $f(x)=x^2$. As $x$ changes, $f$ also changes (according to the specified relationship, $x$ raised to the power of $2$).

Could someone help me clarify?