To be honest, we regularly refactor our architecture at bamboolib in order to make sure that there is almost no gap between what we would love to say in natural language and the code that we need to write.

This resulted in a very stable and clear internal API surface (read architecture). So, literally, all we had to do was adding mount points where users could register their plugins and then include those at render time.

The next day, customers could write plugins just as we did. And, as a matter of fact, all the bamboolib transformations, visualizations, views, etc are just sophisticated plugins that our customers could write themselves because they have access to the same API as we do.

So, no secret sauce except for "good architecture" which is easiest achieved as an ongoing effort rather than an one-off project.