I'd argue it died during the civil war. The removal of secession as an option removes the most powerful check on federal power and set the cards for a collapse of constitutional constraints. Obligatory worth it cuz muh no more slavery (as if the white powers that be were ever really willing to die as a favor to the slaves themselves, one of the most laughable but widespread myths about the civil war).
Difficult to see how legalising secession would improve the US's situation right now.
Slavery was actually bad though and it's fascinating because one reads about all these deeply moral Americans who cared strongly about others, often along Christian lines, and you realise how deeply far the US has fallen culturally since doing something as intrinsically good as abolishing slavery. I mean can anyone imagine American society doing something as deeply good as abolishing slavery? The same people who elected Trump twice? I don't think so.
If we're playing armchair historian, my graph would begin its descent with President Kennedy's assassination. As a country I am not sure we ever recovered.
Everybody loves a good off-by-one error.