Please don't take these numbers as gospel. bStats pages of plugins that have almost all of their users on modern versions will give you a better picture of concrete percentages, such as Chunky or Multiverse, with Spigot at (less than) 10%.
One weird spike is from bStats breaking, the other two are ???, the month of stale data is a month of me not realizing the cronjob was broken.
Negative numbers come from Paper also reporting numbers without plugins and bStats API being only half useful - but the tendency remains correct.