RRD intentionally aggregates data into larger intervals as the data gets older. The monitoring graphs are intended to provide troubleshooting information, not be a high-resolution, historical archive. For that you can query the device using something like cacti or zabbix or a plethora of others. Setting 8 hours x 1 minute resolution is pretty comprehensive. Anything longer than 8 hours and the resolution will be reduced.

@scurrier : check System > Routing > Gateways IS WAN really WAN (an old interface name ?) and now called WAN_DHCP ? (example) ? What are the gateway names ? Is monitoring activated for them ? edit : ls -al /var/db/rrd old, abandoned rrd files (== abandoned interface names) might exist.