The current custom visuals API is overwhelmingly under-documented. The main reason for this, I believe, is that large portions of it are intended to be inaccessible or replaced in the new API currently developed by the custom visuals dev team(Along with a new CLI tool to replace the DevTools).
The following is the github project where they're documenting the new API and related tools, but besides being updated quite slowly, it is currently irrelevant in part, since the new API is yet to be available-
https://github.com/Microsoft/PowerBI-visuals-docs
In the meantime, if you'd like to dive deeper into developing custom visuals using the current API, my recommendations are:
1. Get familiar with TypeScript and the D3 javascript library(Will be useful in the new API as well).
2. Go through code of existing visuals available via the custom-visuals github project, especially that of visuals that implement behaviors/features relevant for your own visuals.
https://github.com/Microsoft/PowerBI-visuals
3. When you encounter interesting/useful/hard-to-understand modules, classes, interfaces and functions in said project, don't be afraid to see how they are defined/implemented and how they're being used by other visuals.
4. Do a lot of trial and error(Probably the most important thing on this list).
5. Search this forum for posts relevant to issues you encounter and don't be afraid to post questions of your own.
Note:
Some API functionality will be available during development in the DevTools and real-time testing, but will not work once you export the visual and use it to your reports. This is due to a security feature called sandboxing, which prevents unofficial/unapproved custom visuals from using functionality deemed exploitable by Microsoft. Such functionality will not be available in the new API.
Tip:
If you'd like to view the source code of custom visuals from the gallery that are not present in the custom-visuals project, you can extract the .pbiviz files(using 7zip, for example) and their TypeScript file will usually be available in its source tree.
