After logfile rotation (ca 06:30 local time), the percentage of IPv6 requests is calculated;
IPv6 destination addresses
----------------------------------- x 100 %
IPv4 + IPv6 destination addresses
The dip in February is due to a design flaw in Squid 5 and higher.
I had to tweak things quite a bit to make Squid behave. See:
Let Squid 5 and higher
prefer IPv6
Due to a RRDtool bug, during DST the graph will show a 2nd October (instead of November) when the '--x-grid' option is used.