Hey!  Funny thing, that happened to me a few days ago.  I ended up having to reboot the machine with a soft power off (press the power button once and hope the machine took the 'off' command.  When it came back, the graphs were working.  in dmesg, I received a bunch of errors showing out of memory.  Someone suggested I run swapinfo, but I had already rebooted by then.  My GUI never came up again, just received a 'this page can not be displayed' error.