I didn’t put this on the graph, but immigration history does make a difference here. Hispanics born in the US have essentially the same high school graduation rate as everyone else, go to college more often than those born elsewhere (somewhat higher than among African Americans), and have comparable rates of attending grad schools as foreign-born Hispanics, both slightly lower than African Americans.
And thanks to other suggestions in the comments, here’re the same data aggregated into three-year chunks, which washes out some of the year-to-year variation:
The decrease in number of people getting graduate degrees does not seem to be a reflection of growing minority populations, as those groups had higher rates of graduate education in the same generation which begins the societal drop in rates of attending graduate school.
I still don’t know what it means, but numbers are fun.