Still no where near the 4 million in the graph. The largest estimate, made by the Discovery channel, was 240,000. Including subsequent deaths by famine and disease. https://en.wikipedia.org/wiki/1975_Banqiao_Dam_failure#Casualties
I don't think the Discovery channel's the ultimate 10 show is the most credible of sources.

I think this is the flood OP's chart is referring to: The 1931 Yangtze-Huang river floods. With an official death toll of 422,420 people. It has nothing to do with the 1975 Banqiao dam break.

>Some Western sources allege that the death toll was between 3.7 and 4 million people based on their own claims of famine and disease.[15][6]

Source [15] is a website with a tiny article. In that article it says:

>in 1931 the death toll was almost four million,

With absolutely nothing to back it up. No sources, nothing. They're just spouting out a number. I don't find this credible at all.

Source [6] refers to a book. This book is about climate. I can't find out which number it claims without buying it. I doubt a 2003 book about climate contains first hand research into a 1931 disaster. It likely quotes another source.

I'm inclined to believe the official death toll over the 70+ year after the fact 'estimates' without any substantial research behind their claims.