The chart state is saved in the session state on the server - when it expires due to a long period of inactivity the control will display this message. You can consider to increase the session state timeout in the web.config of the app:
or enable auto update which does nothing (check out ThinWeb \ Auto Update example) say every 10 minutes or so. That way the session state will be kept alive and you'll not hit the timeout.
We hope this helps - let us know if you meet any problems or have any questions.
Nevron Support Team