Curiosity about how (and how much) the presidents overlapped was spurred by this discussion, looking at which two things coexisted that you might expect hadn't. Charlie Chaplin and 50 Cent, for example. Or the Egyptian pharaohs and the woolly mammoth -- albeit not in the same place. And Herbert Hoover and Barack Obama.
Not that Hoover knew about Obama, of course. Obama was just a kid, living in Hawaii (be quiet, Mr. Trump), and not someone who would attract the attention of a former president. (Obama was no Trace Adams.)
We were curious how frequently the lives of presidents overlapped. So, we figured it out.
Rutherford B. Hayes was alive at the same time as John Adams and at the same time as Dwight Eisenhower. That's pretty remarkable.
The chart is not (and will never be) complete, of course. There are lots of future presidents out there, some of them not named Clinton and Bush. We'll only know whose lives they overlapped with once we know who they are. And the odds are good that none of them will have ever met Barack Obama.