cc @nyuriks
I can link you to similar multiple tree view in Vega : Kibana Vega - Multiple Buckets visualization - how?
https://groups.google.com/forum/#!msg/vega-js/Q2XrYMOVluw/sHlRxtk1DQAJ
Hope this helps, if not revert back, and will try more options.
Thanks
Rashmi