It would be cool to additionally support Wakapi as an alternative to WakaTime. People could then use waka-readme-stats with their own, self-hosted coding statistics backend.

Wakapi mostly provides the same endpoints as WakaTime, so not much would have to be changed. Basically, another variable - something like WAKATIME_API_URL - would be required, that defaults to https://wakatime.com/api, but can be overriden to be https://wakapi.dev/api or https://some-selfhosted-wakapi.com/api. In addition, documentation would have to be adjusted.

If you consider this a useful feature, I surely can implement it and open a PR.