Graph cisco snmp traffic (Data Source Influxdb)

Hello everyone,
I`m new to grafana and i wonder if following query is the best way to visualize my network traffic on cisco

SELECT non_negative_derivative(“ifHCInOctets”)* 8 as “ifHCInOctets” FROM “snmp” WHERE (“hostname” = ‘HE-core-sw’) AND $timeFilter GROUP BY “ifName” fill(null)

Please explain what is the purpose of second time argument for non_negative_derivative(“ifHCInOctets”,10s) i collect my values every 20s should i set this for second argument to this function?? Here is screen of my graph