The sudden downfall in the chart looked so strange, I thought they must have conflated the city of Rome with the Roman Empire. But no, people actually left the city proper in droves, as resources became unavailable and the city fell into overall disrepair:
"The decline of the city's population was caused by the loss of grain shipments from North Africa, from 440 onward, and the unwillingness of the senatorial class to maintain donations to support a population that was too large for the resources available. Even so, strenuous efforts were made to maintain the monumental centre, the palatine, and the largest baths, which continued to function until the Gothic siege of 537. The large baths of Constantine on the Quirinale were even repaired in 443; and the extent of the damage exaggerated and dramatized (according to "Rome, An Urban History from Antiquity to the Present", Rabun Taylor, Katherine W. Rinne and Spiro Kostof, 2016 pp. 160–179). However, the city gave an appearance overall of shabbiness and decay because of the large abandoned areas due to population decline. Population declined to 500,000 by 452 and 100,000 by 500 AD (perhaps larger, though no certain figure can be known). After the Gothic siege of 537, population dropped to 30,000, but had risen to 90,000 by the papacy of Gregory the Great."
https://en.wikipedia.org/wiki/Rome#Monarchy,_republic,_empir...