Where are you sourcing that data from? The graph I linked using data from the U.S. Bureau of Labor Statistics doesn't go back that far, so comparing to 1970 would not be possible.
That's household income. You need to adjust for the change in households with multiple earners. That's why I said the census data is dirty and conflates things. The number of households with both parents working increased from 46% to 52%, so median household income staying flat means median income for individuals went down pretty significantly.