Is it a bug or what am I doing wrong? I am able to reprodecue the issue.
When creating a new device and only use the IPv6 address as the hostname, this results in non working graphs. Although all data is fetched by the poller without issues.
When changing the hostname from the IPv6 address to an FQDN, then my graphs are working.
It feels like that it has something to do with the directory that is made under /opt/librenms/rrd/. When only using the IPv6 address, the directory structure is different as that of an IPv4 address hostname. For example, a device with an IPv4 address gets a directory with 18.104.22.168, the IPv6 device gets an folder with _, all : characters are replaced with a dash. That is the only difference that I see.
Anyway, graphs are working now for my IPv6 node, change hostname from an IPv6 address to an FQDN.
My initial problem came from wheatermap, it could not read from the rrd file, stating it has invalid data. Now weathermap also works for this node on the map.