Without net-analyzer/rrdtool[graph] rrd plugin failed to start.
15 Jul 2014; Jeroen Roovers <jer@gentoo.org> ntop-4.0.3.ebuild, ntop-4.1.0.ebuild, ntop-5.0.1-r1.ebuild: Set USE=graph dependency on net-analyzer/rrdtool (bug #509682).