Your graph should show an increasing relationship. It might also show other features, such as jumps if certain numbers of customers trigger opening another register, thereby shortening waiting time. If we assume that this does not happen, then the number of customers determines the waiting time and number of customers is likely to be the independent variable. As the number of customers increases, the waiting time also increases. We do not know the coordinates of any particular points on the graph, nor do we know the precise steepness of the graph. Nevertheless, we can represent an important characteristic of the relationship by the fact that the graph is increasing.