The Figure depicts the supply and demand equilibrium for port services (point 1), at a given price (P1), corresponding to the quantity (Q1) of port services demanded.
In a typical case, a price increase implies a decrease in the quantity of port services demanded and vice versa. Hence, the slope of the curve concerning price is negative. A change in the quantity of port services demanded, for example due to a price change, implies a movement up along the given demand curve (D1).
Conversely, a change in one or more of the factors that determine the demand for the use of a port and its services leads, over time, to a movement of the demand curve D1 to D2 and the formation of a new equilibrium point. Indicative examples of a move to a new demand curve include an increase in income or population of the economy served by the port.