Dude currently only supports 32bit Octet Counters for easy graphs.
64bit Octet Counters are not supported (HC IfMib)
The 32 Bit Octet Counter will simply roll over more than once within your desired polling interval.
2³² Counter * 8 Bits per Octet / 1000³ Gig /10 sec = 3.436 Gbps
If you shorten the polling interval, you will see higher peak bandwidth.
5 secs will give you 6.872 Gbps
You can nonetheless add custom charts and poll the HC IfMib. But Diff64() also has rollover problems, which are cumbersome to counteract.