I don't think Marshall reversed the axes. The first thing I do with demand
is to show that, for a rational consumer, demand is simply a (marginal)
willingness to pay schedule - which of course relates the (dependent)
willingness to pay to the (independent) quantity. The next lesson is that
market demand is simply the horizontal summation of these individual
curves. The individual producer's supply curve, likewise, relates the
(dependent)marginal cost to the (independent) quantity produced (with the
usual qualifications) ; while market supply is the horizontal aggregation
of these. Reading the demand curve "vertically'," as it were, may seem less
natural than reading it 'horizontally" (for any P, how much will consumers
demand?) but the former is related to the latter as explanans to
explanandum. As far as I'm concerned, Alfred was right !
Kevin Quinn