Price is an independent variable whereas the quantity demanded is a dependent variable. In most sciences, the independent variable is plotted on the horizontal axis or the x-axis and the dependent variable is plotted on the vertical axis or the y-axis. However, economics diverts from this method when it plots an independent variable on the vertical axis and a dependent variable on the horizontal axis.

This convention was popularized by Alfred Marshall who drew the first of the graphs which turned out to be highly influential in the history of Economics. Alfred Marshall applied an opposite approach most probably because, in his introduction of a demand curve in 1879, he considered the quantity as an independent variable and price to be a dependent variable which adjusts regularly to clear the market. However, he was not the first one to draw the demand and supply curves.

Augustin Cournot plotted the price on the horizontal axis in his demand curve in 1838, so did Jules Dupuit in 1844 and Fleeming Jenkin in 1870. Alfred Marshall was well aware of the work of these authors as well as of William Stanley Jevons who plotted the price on the vertical axis in 1871.