Stating Ohm's law as V=IR models the physical characteristics of an electrical circuit, but there are many ways to think about this simple statement. The different ways of thinking about it correspond both to different physical situations and to different mathematical graphs of the equation.
It can be useful to think explicitly about each of the ways that Ohm's law can be represented as a graph. In each case, something is treated as an "input" and something else is treated as an "output."
The standard way of presenting "functions" in math is to treat functions as a way of converting some input value(s) into some output value(s). When it comes time to apply this concept to the physical reality of electrical circuits, you are left with choices: Which quantity is the input? What is the output?