HI
Thanks for your interest in plugin development for Kibana. Here is a detailed tutorial for plugin development:
https://www.elastic.co/guide/en/kibana/current/plugin-development.html
The contributing guide is a good starting point.
Here is sankey visualization guide : https://www.elastic.co/blog/sankey-visualization-with-vega-in-kibana
Here's a good keynote on plugin development II also found this to be useful : this video
If you need more info plz do let us know .
Thanks
Rashmi