It displays the number of instances (in our famous "yearly grid") of player-seasons where a hitter has amassed 20 or more triples.You can see what's happened to triples over the course of baseball history by examining the frequency of peak season performance.
There have been 113 player-seasons in which a player has hit 20+ triples. 97 of those occurred before 1930.
The chart also shows that the triple was on its way out in the deadball era. The primary culprit is shortened outfield dimensions that came into play as the first wave of modern ballparks were constructed.
Broadly speaking, every change in ballpark configuration has served to minimize the occurrence of triples. That's documented in the fact that over the past sixty-five seasons, we've had exactly seven players who've hit 20+ triples in a year (as opposed to 106 in the first sixty-five years since the schedule was lengthened to something resembling what it is today).
It's instructive to look at the same yearly grid applied to the home run. We've used 40+ homers here because the ratio of that figure to the home run record (73) is almost identical to the ratio of 20+ triples to the 3B record (36, set just over a century ago--in 1912--by Owen "Chief" Wilson).
Adherents of what is often called a "sillyball era" will be buoyed by what they see on this chart, where two decades--1990s, 2000s--account for more than half of the 40+ HR seasons in history.
It's looking as though we are moving back to the patterns in place during the 50s and 60s--the last seven seasons have produced 24 40+ HR seasons, or a tad above three per year, which tracks best with those decades. But the falloff of 40+ HR seasons, while warranted by historical propriety (which, in baseball at least, has some hope of being measurable...), is increasingly accompanied by an unease amongst so-called "thinking fans."
Rather than worrying about the game becoming too boring because of a return to "HR sanity," those folks (whom we'll take on at greater length in the not-too-distant future...) might want to consider joining a crusade to increase the frequency of triples.
