I've seen something like that before - not Netflow obviously - but I've seen it. Now I'll just have to remember where :-)
Oh, it was the other day when I was looking at Tobi Oetiker's website. And ad on his site for this guy :
http://community.zenoss.org/index.jspa
I have been meaning to download and try it out. When I took a quick look at features the other day I think it does this sort of thing.