The rise started before 1950, rose the most rapidly from 1960 to 1970, plateaued in 1980, and then collapsed moving towards 2010.
https://www.ncesc.com/geographic-pedia/at-what-age-do-serial-killers-start-killing/
As previously mentioned, the typical age range for serial killers to start killing is in their late 20s to early 30s.
So figure that the people killing were maybe maybe late 20s to early 30s in late 1950s to 1970, when the numbers were exploding.
That means people born in ~1920 to ~1940; the serial killers probably were mostly born in the interwar period, between World War I and World War II; born in the Roaring Twenties and then the Great Depression.
https://en.wikipedia.org/wiki/Millennials
Going based on the generations there, that would have mostly been the Silent Generation.
The period of rapid increase was only about twenty years long, so it's really only about the length of one generation (though that doesn't mean that it need nicely align with the "generational cohorts" thing).
The Boomers were already falling off.
By the time Generation X rolled around, the spike would already have been done.
Millennials were born between 1981 and 1996, long after all this happened.
And one other point -- remember that the graph is of absolute, not per-capita numbers. According to it, in 2010, we have numbers in absolute terms comparable to about 1955. But that's in absolute terms.
https://www.populationpyramid.net/united-states-of-america/1955/
In 1955, the US population was about 106 million. Today, it is 334 million. That is, in per-capita terms, 2010 is somewhat-lower than any period shown on the chart. It's not just low, it's lower than it's ever been.
Now, all that being said, I'm not sure how they measure the number of concurrently-active serial killers. I would imagine that things like the advent of DNA evidence, buildup of fingerprint databases, and other changes in criminology probably have changed things; one might have assumed that a serial killer was responsible for a copycat/similar crime, or perhaps vice versa in different conditions.