Looks pretty level to me - looks different than the wall poster my financial guy has on his wall.
The drag of the Vietnam war on the economy perhaps?
Cold war spending draining things?
Energy crisis (both of them) seemed to have no effect, why?
Nixon's inflation, perhaps making real growth look stagnant?
Looking at it again, Basic Chart of the Dow Jones Industrial...................bet the other charts/markets are more upwardly moblie by comparison. This would tell us a lot, the comparison that is.