That’s not what “hypothetical” means. Do you believe there are such things as genera, species, and generations? Then saying “let this line represent a thousand generations” is not hypothetical.
I repeat, can you do even that much with the degeneration chart?
The Y axis represents the genetical richness of the original created kinds. The X axis represents the loss of genetical richness over time. Same difference.
==That’s not what “hypothetical” means.
Darwin’s entire chart was hypthetical = conjecture. None of the species or generational time frames pertained to actual data. They all existed in his own head. He was using the chart to help the reader understand an idea. In this case, natural selection.