In this search: (for example)
index="_internal" source="*metrics.log" group="per_host_thruput" series = ( result of another search indicating a specific index ) | chart sum(kb) by series | sort - sum(kb)
So that it will just display all the hosts that are related to a specific index.
You can simply use a subsearch in "[ ]" Like this:
index=_internal source=*metrics.log group="per_host_thruput" [search something that will result in series=something] | chat sum(kb) by series | sort -sum(kb)
Hope this can help you!