There is no rule and it doesn’t matter which you let be the x axis and which you let be the y axis. The graph will look ‘sideways’ but the final answer will be exactly the same.
It is now irrelevant anyway because, as I say in my lecture, you cannot any longer be asked to draw the graph in the exam. However it is still important to understand it because you could be given a graph already drawn and be asked to interpret it and to use it.