The data from an individual demand schedule or a market demand schedule can be graphed as a demand curve. Price (per unit) is shown on the vertical (or ''Y'') axis, and the quantity demanded per unit of time is shown on the horizontal (or ''X'') axis. The demand curve relates the price (per unit) of a good or service with the quantity of that good or service which a consumer is willing to buy (in a certain period of time).

In the diagram, we can see this relationship. If the price of tomatoes is $5 per kilogram, consumers will buy 1 kilogram of tomatoes per week. If the price of tomatoes falls to $4 per kilogram, two kilograms of tomatoes will be bought per week.

If we add up all the demand curves of all consumers (for a particular good or service), we can prepare a market demand curve.