Are you referring to the Dashboard "history" graph ?:
Several others have had issues with that display lately.
Have you tried a hard rest of the modem ?
While not as detailed as the hourly above, you could get some info from the Download Status Meter usage history readout:
As a last resort you could try the display found here:
Be advised that this page is no longer supported, shows gross (vs. net) usage and is listed from the perspective of eastern time:
Mine also works good but really needs a "time stamp" to help align a user to the period.