Please perform the following troubleshooting steps, and this should resolve your issue:
Stop the Wowza Streaming Engine service.
Delete all the files in [install-dir]/stats folder. These files would be re-generated once you have restarted the WSE service.
Reset the timezone on your server. You can also try disabling the option to set time automatically (if it was originally set to enabled).
Start up the WSE service again and login to the WSE manager.
Test playback on one of your streams.
You can use the test players on the Wowza website (http://www.wowza.com/testplayers) making sure to start up playback on several tabs to simulate multiple client requests.
You may have to refresh your browser initially to see the updated chart.