I think the more interesting here is the intercountry comparison. For example, the richest group in Japan is slightly less satisfied than America's poor. I wonder if that is reflective of the Japanese being unhappy, Americans being happy, or an incommensurate comparison whereby we understand satisfaction to mean different things. Also, for the mathematically ignorant, a lin-log plot showing a linearity implies a logarithmic relationship, which means that there is a diminishing return on happiness with new money. So it appears the the trope that a little money makes a poor person a lot happier than a lot of money makes a rich person does, in fact, appear to hold across all the countries surveyed, even if there isn't a hard cap beyond which one doesn't get happier with more money.