My Ntop doesn't have that. Do I have to activate something in the configuration menu?
BTW if I restart the NTOP services via pfsense web console, all my configuration that I made on NTOP webconsole are lost...
What configuration where? Sounds like to me you don't have it installed to the HDD. If you are running version .93x then that is probably your problem with Ntop. Reinstall version .87 as that is the only version I can guarentee it will work with. Perhaps .90 will work too as I have had it working on that version by upgrading from .97etc.
You have enable rrd tool. You said above that you had all the traffic statistics per ip etc, now I know you are just talking about the statistics shown in numerical form only. The statistics I am talking about are avialable in graphical and numerical form and you can choose 1 hour 6 hours 12hours 1day 1month 1 year. To view the history of each IP or a total.
Once you enable rrd tool when you click on an individual IP to view extra information about that person you will find a columb "Historical data" and a little Icon of a graph. It's easy to miss so look carefully for it.
Let me know how you go. In .93 when you enable rrd tool you will probably have an error