This thread is for anyone wanting to write an external (or internal) plugin for Bytecode Viewer.
First off, take a look through the Java docs for the API package - https://the.bytecode.club/docs/bytecode-viewer/the/bytecode/club/bytecodeviewer/api/package-summary.html.
Once you become familiar with the API, I suggest you take a look at the internal plugins for an example of what a real plugin should look like - https://github.com/Konloch/bytecode-viewer/tree/master/src/the/bytecode/club/bytecodeviewer/plugins.
Now that you've got a good idea of how the plugin system was designed, here are some skeleton scripts to get you started - Groovy, Ruby, Python.