Zero chance that that graph shows the change in real estate prices in China. Even outside of Tier 1 cities, where by any measure, there has been a substantial fall - but not a 75-80% one ... (and most of the value measured in yuan, is in Tier 1 and Tier 2 cities).

I'm guessing it shows real estate stocks? That would make perfect sense.

(Of course the usual caveat - you can't trust any statistics coming out of China, generally - if this is a stock price chart (aggregate over several companies), it may be coming out of HK rather than the mainland market, and generally that's substantially more trustworthy).

But without a link to the source or a mention of what the y axis actually is, I can guarantee people (most?) responding to you in the thread are assuming that it shows real estate prices.