I've posted a suggestion in the issue tracker to allow nagios to monitor both the used bandwidth and the usage of backup space. If like me you'd like to check everything on one page rather than having to login to both nagios and the panel then please take the time to vote it up so that Andy can tell how many people would like it.
https://tools.bitfolk.com/redmine/issues/41

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Robert Gauld
http://www.robertgauld.co.uk