Demand curve

A demand curve makes a conditional relationship visible. In the conventional diagram, price is on the vertical axis and quantity on the horizontal axis, even when the algebra writes quantity as a function of price. State the chosen form so slope units and intercepts are not misread.

Movement and shift use different evidence

A change in the good's own price selects another point on the same curve. A change in a held-constant determinant can shift the curve, meaning a different quantity is demanded at each relevant price. A before-and-after sales pair alone cannot establish which occurred because both price and nonprice conditions may have changed.

A straight line is a local teaching approximation, not a claim that willingness to pay follows one linear law at every price. Intercepts outside the relevant domain may have no behavioral meaning. Estimated curves also inherit sampling, measurement, functional-form, simultaneity, and identification limits.

The curve organizes a counterfactual: what quantity would buyers choose at alternative prices, other modeled determinants held fixed? It is not a ledger of observed sales, a causal estimate by definition, or a recommendation about price.