sudo apt-get install ntop sudo install -d /var/lib/ntop sudo install -o nobody -d /var/lib/ntop/interfaces sudo install -o nobody -d /var/lib/ntop/rrdTo test, start "sudo ntop" (probably in "screen"), then open the web page http://localhost:3000
To activate the RRD plugin, select Plugins / Round Robin / Describe, then click the red "no" to become a blue "yes." You'll see messages in the terminal where you started ntop.
After a few moments, you can view RRD graphs. One is in Summary / Network Load.