You may also want to take a look at http://host:8000/en-US/app/sos/license_usage_30days on your license master, that gives you a volume per day splittable by index.
Also available under http://host:8000/en-US/manager/search/licenseusage not requiring SoS.
...app/sos/indexing_performance lets you pick any indexer and can split by index, .../app/sos/indexing_distributed lets you split by indexers but not by index... but this being Splunk, anything can be extended. Taking the queries from those two views, you could build this:
`set_internal_index` source=*metrics.log group=per_index_thruput [inputlookup splunk_servers_cache | search server_role!="search-head" | search server_role!="*forwarder*" | eval host=sos_server | fields host] | timechart minspan=30s per_second(kb) AS KBps by series useother=false limit=100