traffic directions in billing

For now, observium bills "what is greater" of in and out traffic on the interface(s).

Would be nice, if it would be possible to select direction of traffic for billing for each interface in bill, with following options:

• bigger (as it works now)
• in
• out
• in+out (sum of both)
• in-out (deduction out from in)
• out-in (reverse)