The following splunk query will show a timechart of network traffic received over a period of time for any host specified (make sure you edit the query to specify a host, this one defaults to all). The query also converts the default value of Bytes to Megabytes.

sourcetype="Perfmon:Network Interface" (host="*")  counter="Bytes Received/sec"| eval MB=(Value/1024/1024)| timechart avg(MB) by host