In addition to the transition overseas of other commentors, I would add the lack of socialized health care in America as a major factor. By placing health care costs on employers, employers are encouraged to hire part time labour or cut wages to compensate for health care contributions. Most organized labor disputes now hinge on, not wages or hours, but health care benefits.
The second trend that is frequently overlooked is the entry of women in the workforce. Wages were based on single-earner households. Instead of two wage earners doubling income, they split the wage in half. Wages didn't stagnate - they split, like a stock. The buying power of a two-earner household is now equivalent to a prior single. If you take your chart and double the wage, you'll see it roughly realign.