Pat just had his car tuned, but he was still dissatisfied with his gas mileage. Therefore, he decided to try a different gasoline and assess the gas mileage. For two weeks he purchased his present gasoline, Cheap-o, and noted the miles per gallon that he received. The following two weeks, Pat purchased a better brand of gas, Shell, and noted those miles per gallon. If Pat graphs the results of his experiment, what variable should he place on the Y-axis?