I've found a bug in "Bandwidth Usage Report". The data from the montly table is shifted by 1 day:
I think it is related to timezones, since when creating the "rrdtool fetch" command the local time is used for the starting and ending arguments (1514775600 and 1517454000 as it can be seen from the picture). However, RRDs do their consolidation based on GMT/UTC timezones. If your time zone is <0, a point is missing from the command output and causes the offset.
Another consequence of this behavior regarding RRDs is that the "daily usage" actualiy considers the bandwidth of multiple days (but I understand that it's something that can't be fixed easily), depending on your timezone.
Thanks, Gonzalo