I think the relationship between income and the happiness of an individual is in line with the principle of diminishing marginal utility.
Money can help to increase one's feel of happiness in almost every way that a person can be helped. Money can bring a person better life condition and make most of the problems in life easier to solve. Money can even bring more feel of value to the person by helping others.
At this stage, the relationship between income and the happiness of an individual is a positive correlation, which means the more money a person have, a happier life a person can live.
But if the curve go further, a person already have billions of money, he /she is now pursuing meaning of life and inner peace, the curve will go flat at this stage. Because at this stage, money can not bring answers to a person.