NTP RRD graphing fails to recognise that offset can be negative
The definition of the offset date source in /etc/inc/rrd.inc fails to recognise that offset can be negative, meaning all negative offsets are recorded as unknown.
The attached patch resolves the issue when /var/db/rrd/ntpd.rrd does not exist.
There is no obvious way of fixing the issue retrospectively.
rm /var/db/rrd/ntpd.rrd at the command line following by 'Save' in the Settings tab of Status: RRD Graphs is recommended.