Once a Year Occurrences in MLB

I was thinking about home runs, as one does, and wondered what the standard for a great season is. A number of home runs that tends to occur only once a year on average seemed like a decent way to think about greatness. My gut said it'd probably be around 50.

From that, several other questions followed. How would those seasons be distributed across all seasons? How far back in time should I go? I bet a bunch would be during the steroid era. What about other eras? How much of an outlier are the really great seasons, like Bonds' 73 HR in 2001? What about other stats? Would be cool to evaluate those as well.

We'll be taking a look at a set of hitting and pitching stats, first plotted by season (while grouping stat values per season in a tooltip for each plotted item) and then by stat value (while grouping seasons per stat value in a tooltip for each plotted item). We're looking at all seasons from 1901 (commonly considered the beginning of "modern" baseball) to 2023 — 123 seasons.

I used FanGraphs to pull the top 123 season performances from 1901–2023 for each stat, which usually resulted in a few more due to ties at the end of the range. Those values are plotted on each chart, and weighted by the number of occurances per year or value.

For the season-based charts, I added markers for MLB eras to add context, and for the stat-based charts, I added the min and max values at the ends of each row.