Hi Mike,
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:
<configuration>
<system.web>
<sessionState timeout="20">
</sessionState>
</system.web>
</configuration>.
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.
Best Regards,
Nevron Support Team