To analyse a graph you are meant to determine a general trend, relating the results of an experiment to the hypothesis as well as to form. For example you look at all the values in a line graph and you are yo predict one you are to find its common increase or decrease.
If all possible vertical lines will only cross the relation in one place, then the relation is a function.This works because if a vertical line crosses a relation in more than one place it means that there must be two y values corresponding to one x value in that relation.
