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