Don’t forget mainland China had also been looted for a century and in a state of war/occupation for the 75 years preceding this graph.

That's a major contributing factor as to why China was so relatively underdeveloped as of the revolution, yes. And for a sense of just how little industry they had at that point, in 1950 China's entire steel production was less than that of Russia in 1917 (and the comparative underdevelopment of Russia is often something that left anticommunists point to when saying "lOoK hOw It WaS dOoMeD fRoM tHe StArT! sHamE iT dIdN't HaPpEn SoMeWhErE cIvIlIzEd InStEaD!"), and a lot of the heavy industry that they did have was aging factories built by the Japanese in Manchuria during its occupation (and using those factories was an ideological sticking point for the CPC, because they basically couldn't be operated without mimicking the sort of abusive and extractive methods that they were built for).

The old aristocracy also contributed to the problems, particularly when it came to education which was systematically denied to most of the population (and as a side note, that created some interesting policies on the part of the CPC in the early 1950s: despite the popular idea of mass purges of elites, bourgeoisie, and landlords a lot of them wound up cutting deals with the CPC to hand over their capital and land to the state in exchange for bureaucratic positions since they were literate and the newly formed state desperately needed literate bureaucrats immediately).

And to your second point, there are now cities in China larger than HK or Singapore that didn’t even exist when this graph starts.

Yeah, I'm just curious how those numbers would line up, since China's cities specifically would probably be starting at or above the point Hong Kong started at, and probably growing faster except for during certain periods. China's development has just been so uneven between rural and urban areas that mashing the stats for both together creates a number that's fundamentally dishonest for both.