As the price of a product increases, the quantity demanded by buyers decreases. This is reflected by a demand curve that is plotted with quantity demands on the x-axis (horizontal) and price on the y-axis (vertical):
When some variable other than the price of the product causes demand to change, it is referred to as a demand curve shift.
Positive shift—An increase in demand at each price (the line moves to the right)
Negative shift—A decrease in demand at each price (the line moves to the left)
This is an illustration of a positive demand curve shift:
Certain factors have a direct relationship to the demand curve for a product (an increase in that factor will cause the demand curve to have a positive shift to the right):