After the price change the price line shifts from PQ to PQ′ and the consumer reaches equilibrium B…

In the diagram, Tomato (Y) is measured on the vertical axis and Potato (X) on the horizontal axis. The outward rotation of the budget line from PQ to PQ' (with vertical intercept P unchanged) shows that the price of potatoes has fallen. When the price of potatoes falls, the consumer's optimum moves from A to B, which lies to the left of A—indicating that the quantity demanded of potatoes decreases as its price falls. A good whose quantity demanded moves in the same direction as its price (violating the law of demand because a strong negative income effect outweighs the substitution effect) is a Giffen good .