arm bandwidth graph reset by sighups
arm uses the tor process' uptime to determine if all the values in the state file can be trusted to belong to this tor instance or not. Unfortunately issuing a sighup evidently resets the uptime reported by ps, making arm skip the bandwidth graph prepopulation for the following day.
Sebastian pointed out this is especially a problem for Debian since it has a sighup issued daily (meaning we'd never prepopulate the bandwidth).