I was having trouble to get my RRD plugin shows the network load graph in Ubuntu, although the apt-get should resolve that automatically. The following is the method that I gathered from various online sources, so that I can refer back whenever I need to do a fresh install.
sudo mkdir /var/lib/ntop/rrd
sudo mkdir /var/lib/ntop/rrd/graphics
sudo mkdir /var/lib/ntop/rrd/flows
sudo mkdir /var/lib/ntop/rrd/interfaces
sudo mkdir /var/lib/ntop/rrd/interfaces/eth0
sudo mkdir /var/lib/ntop/rrd/interfaces/eth1
sudo mkdir /var/lib/ntop/rrd/interfaces/ppp0
sudo chmod -R 775 /var/lib/ntop
sudo chown -R nobody:nogroup /var/lib/ntop/rrd
*note, for eth0, eth1, pp0, you may need to replace that with your own preference, some machine, such as Dell Latitude D430 use wlan0 for its Wifi card.
I also could not understand, this problem was let ignored for several releases, and no action being taken so that ntop could work just straight away with RRD.