I work in this area and I really dislike it when happiness is plotted against income only on a log scale, as is done here. The log scale effectively conceals the flattening-out of the relationship, which is the thing that shows you how progressive taxation + redistribution make the average person better off.

Whether it's a straight line up to $75K followed by a flat line from there on, or a curved (log) relationship all the way up, seems to me to make rather little difference.