> Price index do not capture the reality of most people cost of living. They don't capture the family that have to buy margarine instead of butter, because butter became too expensive, while margarine became cheaper, Weight both items as the same, and voilá! no inflation, increases in the price of butter were cancelled by cheaper margarine, fuck you if you prefer butter.

I don't know how this works in other countries, but in the US the weights do not change month to month like this. If consumers keep making this choice over a period of years the index will eventually get reweighted, but reweighting only happens once every few years.

(Of course, if you were an official put in charge specifically to cook the books you could just reweight on a monthly basis like you describe, which is something that might well happen now...)