Legacy Cumulus 1 release 1.9.4 (build 1099) - 28 November 2014
(a patch is available for 1.9.4 build 1099 that extends the date range of drop-down menus to 2030)
Download the Software (Cumulus MX / Cumulus 1 and other related items) from the Wiki
The monthly total is calculated from the daily totals in dayfile.txt, and the usual reason for it not being correct is a problem in that file. But the graph is also drawn from the same data, so it's strange to see one correct but not the other. Try restarting to see if that fixes it. You could also look in the diags files to see if there is an error message.
If you can't work out what the problem is, zip up the diags folder together with dayfile.txt and attach it here.