git-of-theseus-line-plot: incomplete diagram

This command doesn't pick up all authors, in my case it only detected 1 out of 6 authors in a project.

```bash
# list all authors in a project
git shortlog --summary --numbered --all --no-merges
```

```bash
# stack line diagram only features one author
git-of-theseus-line-plot authors.json
```