b/c if lets say it was $2 before the price was doubled after it waas doubled it would be $4 if it had been risen by 50% it would have been $3 but if it waas risen by 100% it would be $4
So the y value of the vertex of the parabola that models the data should be the left hand side of the chart; Number of $2 increases in price. So however your graph is, it should make sense.