Traffic Graph Stop Working (Solved)

[beafool]

On the WAN interface only, the Traffic Graph refused to plot the realtime traffic graph all of the sudden.  (see attached picture)  Other interfaces have no such problem.  Reboot the firewall does not solve the problem.  Any idea to make the graph appear again?

I am using the 2.0 64bit release.  Thank you in advance.

[Edit]–Restart the client computer fixed the problem.  The problem was probably caused by browser (thunderbird) process stucked in the background without me knowing it.