In the mid-nineteenth century, Fleming Jenkin showed the price correctly on the x-axis of the Cartesian coordinate diagram. For some reason [ask a Marshall specialist], Marshall put the price on the y-axis in his footnotes to his Principles. It is my understanding that the convention became one of putting the price on the y-axis because Marshall's Principles then dominated the English-speaking world's introductory economics courses for several decades after its publication--unless this is one of those intellectual history myths. Tell your student that it doesn't really matter to violate the geometrical convention in two-dimensional space. When you move to n-dimensions, there's no problem because there's no picture. Sam Bostaph