I read somewhere (sorry can't find the thread) about checking a users datatransfer.

Can't i just check it at the control panel "siteusage" and select the virtual site i want to see ?

is there a way to let the graphs show a certain period of time, like januari ?

I was never a big fan of the CP stats. You might want to install webalizer for RaQ3/3i (ftp://ftp.cobaltnet.com/pub/experimental/raq3/webalizer-auth-1.30.04.i386.pkg) which looks and acts much better.

webalizer for RaQ3/3i README file (ftp://ftp.cobaltnet.com/pub/experimental/raq3/webalizer.README)