That chart tracks the stock market. It has nothing to do with unionization. It has to do with the massive boom in the stock market after the 1970s and the pay packages that began to be linked to stock performance. The stagnation in the middle perfectly coincides with the stock market stagnation covering nearly two decades. The wage increase toward the end of the chart perfectly coincides with the stock market lift-off that began in the 1980s. The huge drop in the early part of the chart, is the great depression hammering the stock market (ie the capital class and what they earn from such investments, as corporate profits evaporated and the stock market crashed), that also had absolutely nothing to do with unions.