Nothing. see the attached screen shot.
I know this sounds stupid. But let's rule out a wonkiness with the web browser; it has happened before with the GUI. If you're using Firefox, try Chrome. If you're not using Firefox, try Firefox.
Number 2: Go into /var/db/system You should have a directory in there that is rrd-{abunchofcrap}. In there, in most cases, there will be a subdirectory called localhost. In there will be all of the reporting-tab raw data. Can you verify that these files are present, and that they are timestamped up-to-date?