A quantity 'y' is said to be proportional to another quantity 'x', if both the quantities increase and decrease simultaneously. An ncrease in 'x' will resultin increase in 'y' and a decrease in 'x' will result in decrease in 'y'.
Generally, in such cases, 'y' is said to be the dependent variable and 'x' is said to be the independent variable.
In order to judge the relationship between the variables, the points are plotted on the xy plane, taking the independent variable 'x' on the horizontal axis, and the dependent variable 'y' on vertical axis.
However, the graph is already given in the question, which can be referred directly.
Now, observe that as you go from left to right on the curve, you will get higher values of 'x' and correspondingly higher values of 'y' as well. It means that an increase in 'x' is reflected on quantity 'y'.
Now check for the decrease. As you go from right to left on the curve observe that quantity 'x' decreases which results in decrease in quantity 'y'.
Thus, it is observed that both the quantities 'y' which represents the Points Scored, and 'x' which represents the Minutes Played increase and decrease simultaneously.
Since they fulfil the necessary condition of proportionality, the quantity Points Scored is said to be proportional to the quantity Minutes Played.