This graph shows how much an average individual demands of a good at different prices. For simplicity, we assume that this the the kind of good that consumers spend a constant fraction ( \alpha ) of their income (I) on this good.
This graph shows the overall market demand for this good if there are N consumers who on average have the demand curve shown on the left. Thus, at any price P, if the quantity demanded by each individual is (on average) q^D(P), then the total amount demanded by all N consumers is: