I would say:
- First Elixir book published (I think it was Programming Elixir by Dave Thomas) but I am not too sure about which one was first (More research needed).
- Milestone of 1.000 contributors to the Elixir programming language (Not sure if there are tools that keep track of this, It was mentioned in an Elixir Conference by James Fish)
- In addition to the first commit, there is a long hiatus and then a lot of code changes, that is when Elixir was sort of rewritten and it is what we now know. You should include this one I guess. You can see this in the graphics in the GitHub repo.
- The date that Plataformatec is aquihired by Nubank.
- If you are going to include first commit of external projects, it should also include when 1.0.0 was released as well.
- Add first commit of ExDoc.
- Add first release of ExDoc with the design that we now nowadays.
- Launch of Hex.pm
- Launch of Hexdocs.pm