Price or Quantity on the X Axis?
There is a small debate over whether price or quantity should go on the x axis of the supply and demand graph. Historically, quantity has gone on the x axis, since that’s how Alfred Marshall, an early pioneer of supply and demand analysis, drew it.
The convention in mathmatics is to put the independent variable on the x axis and the dependent variable on the y. When you have variables in causal relationships, the variable on the x axis causes the variable on the y axis. Neither price nor quantity satisfy these criteria. Prices do not cause quantities, nor do quantities cause prices. Both are jointly determined by the supply and demand curves. The domain of the function is the two curves, the range is the Cartesian coordinate which corresponds to a price and a quantity.
f(supply, demand) = (P,Q)
The Vertical Line Test
Demand curves are almost always downward sloping, but supply curves are all over the map. As a function can only have one output for each distinct input, the vertical line test helps to determine which variable is the domain of a function and which is the range.
Both of these supply curves are valid according to economic theory:
Note: “Backward bending” supply curves occur in labor markets when, at high wages, people may decide to work less and enjoy more leisure time.
Price should remain on the vertical axis, not because it is the dependent variable, but because it may be mistaken for the independent variable. No one thinks that quantities are causal, but some people do mistakenly think that prices are. Putting P on the vertical axis is a subtle reminder that prices don’t cause things.
A bit of history on the subject from the Richmond Fed.