As with all things MRTG, its just a matter of finding/using the right OID or modifying the target to suit what you need. MRTG essentially collects numbers and graphs them. By default it collects the numbers via snmp, or you can write your own scripts (sh, bash, perl, whatever) to output the numbers that MRTG graphs for you.
If you want a cumulative graph or just to display the same data in GB per second all you need to do is add a new target or modify the one you have (your original post is not clear to me in that way).
For example if you wanted to keep the one traffic graph but have it show in GB per second also, copy the target for that graph, and then apply the appropriate math to the target. That number is pretty big so you may be thinking something different.
Depending on what you are actually looking for you might use the gauge option for the target. If you are looking for 95 percentile there are lots of hits on Google for that.