I've installed ntop on two machines at the same time, my old laptop and a new server, both running Ubuntu 14.04. My laptop showed the pie charts correctly through the web interface, the fresh installation on the server showed no pie charts at all. Switching from Firefox to Chrome didn't solve the issue. I've also restarted the service many times, I've tried to diff the two access.log files to see if something was missing, to no avail.
Finally, going through Firefox dev tools, I've found that all the <div> containing the pie charts were totally empty. Given that the "jquery.jqplot.min.js" (and co.) scripts could be successfully GET, I started to skim through "/usr/share/ntop/html" and I found that on the server the "jquery-1.7.2.min.js" symlink was broken.
Morale of the story, ntop should have a dependency against "libjs-jquery", otherwise no pie charts are displayed through the web interface (a real shame). Surprisingly enough, none of the packages for Ubuntu and Debian seem to have "libjs-jquery" as dependency.
Status changed to 'Confirmed' because the bug affects multiple users.