If your Munin 1.4.7 apache_accesses and apache_volume plugin graphs are blank, you could be experiencing the following problem.
If munin-run looks like this:
$ munin-run --debug apache_volume [...] volume80.value U
and the apache error_log looks like this:
[warn] (101)Network is unreachable: connect to listener on [::]:80
- disabling IPv6 on your server
- or add Allow from ::1 for /server-status
- ExtendedStatus On is required.