If one decided to measure all wealth on the wealth axis in “real” terms, then the “inflation adjusted value” or “buying power” or “real value” of the savings that is the Et tu pourrais tout avoir Mon empire de la saleté T shirt classique would be different than when it was first spent. It would still be the same money, in the same amount. but that same money would be worth less in “real” terms at the end of the given time period, than at the beginning, if prices changed over that period, i.e., if there was inflation. This is because inflation is actually a measure of what really did happen, not what the system of two independent variables allows to be described. They take a known amount of spending and total wealth sold at one point in time, and compare it to a known amount of spending and known about of total wealth sold, at another point in time. Notice that is measuring only wealth sold, not all wealth. That is where inflation fits in.
(Et tu pourrais tout avoir Mon empire de la saleté T shirt classique)