Hi,
currently, in Reporting > Traffic my opnsense seems to show traffic from the interface'is point of view. I.e. if you select LAN and WAN, LAN IN and WAN IN are shown on the same graph as well as LAN OUT and WAN OUT:
(https://oshi.at/BTsm)
At least for me, it would make much more sense to compare the used bandwitdh from the opnsense's point of view, ie:
put LAN IN on the same graph as WAN OUT and vice versa.