Monitor graphs - 2 x BW if Area selected

[bingo600]

I'm running latest image:
2.4.0-BETA (amd64)
built on Sat Jul 01 21:47:06 CDT 2017
FreeBSD 11.0-RELEASE-p10

And playing with Status->Monitoring , and see 4Mb/s utilization if i use graph Lines (seems correct).
If i switch the graph to Area the utilization doubles , as if the "out total" it self is added to the out.

Is that a "feature" or "bug" , or me totally missing the concept here ?

See attached images

TIA

/Bingo