A mathematical curve, drawn on a graph, that represents what the demand for a commodity would be if its price ranged anywhere from zero to infinity. The point at which it intersects the supply curve for the same commodity supposedly establishes the price of the commodity in a free market. (See supply and demand.)
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There is a demand-schedule, which, plotted, would give a demand-curve.
It is enough that the demand-curve be recognized as expressing economic significance, and diminishing economic significance.
The two independent causes, then, are summed up in the supply-curve and the demand-curve.