re: What minimum salary would be required to be part of the middle class?

Obviously there are outliers, but typically those who are making over 100k have some sort of education post baccalaureate. If you have a bachelors degree you probably make more than 50k, etc...

Outliers such as plant operators, entrepreneurs, and off shore O&G workers (others I'm missing too) can bring in substantial (100k+) salaries, but that is not common for the uneducated population.

I don't think anyone is thinking that that chart is carved in stone, but rather what is typical for those educational levels.