I think going back a few decades waters down the data. I don't think our justice bureaucracy was as corrupted back then as it is now. I think this is a new development in our government, and I think the recent trend is bad, and the historical performance is irrelevant.
Nonetheless, it would be interesting to see data about campaign aides and executive branch staffers investigated/interviewed by the FBI in the past 10 years. I think that Obama's aides weren't investigated, and HRC's aides got immunity to get them to talk, with no indictments. I think that Trump aides got indictments to get them to talk. I think an investigation was started with out evidence as an insurance policy, per Lisa Paige.
But, whatever. It is what it is. I can't change anything. I'll still have a beer in my recliner Friday night.
(Side note: Funny that your data is opposite that of golfkrzy's meme.)