Get back Combined Graph feature

Till 3.1 version Graylog included really great feature [Combined Graph](https://docs.graylog.org/en/3.1/pages/queries.html#field-graphs), but it's gone since 3.2

Please consider to get it back, as it's really useful for issues analysis.

My usecases:
1. Based on network quality clients can request resources with different resolution, so such graph helps to identify when something goes wrong on the network
2. Estimate how many clients use old API over new API

Initial discussion:
https://community.graylog.org/t/combined-graph-and-3-3/15564