Marginal Utility Meaning
That's an entertainingly specific question to ask! I don't know the answer, but my thoughts are this.
You're looking at an aggregate demand curve, which is a result of all the demand curves of individuals in this economy. Now, individual demand curves are fairly easy to understand. Each individual takes prices to be fixed, and then they maximise utility. When the price changes, for instance because the economy moves from point P1 to point P2, everyone adjusts their consumption behaviour. How much they'll change their consumption is then a question of their utility functions, and hence is specific to the product and to the economy.
So, this is my answer: I think either could be true, it just depends on the product, prices, and utility functions.