How do I add a custom Vega transformation?

I would like to create a custom Vega transformation, specifically to put time ranges into arbitrary buckets such that they never overlap in a given bucket (basically to show the duration and disposition of a set of tasks that have been processed). I've looked over the existing transform code, and read through the Vega extension API docs, and this sounds doable.

What I can't figure out, though, is how to actually call the Vega extensibility API. Are there any examples of similar extensions or plug-ins that do similar work? Or can you point me to how I could gain access via some other channel? I've been unable to find anything related on GitHub.

@moxenb
unfortunately you can't use the vega extensibility API right away from the Kibana Vega editor.
What you can do is you can build a Kibana visualization plugin https://www.elastic.co/guide/en/kibana/current/development-getting-started.html to build your own set of functions/vega extensions.
On the other end, if you can be more precise on your use case and provide me an example I can check what can be done directly in vega if possible