Why this pattern? From for example Jan to July 2018 there are 27 "cycles" - probably weeks. They go from say 18% to 22% in a week in a regular pattern. Peaks are sharper than the valleys. Are those weekends? Is there more use of IPv6 in the weekend?
Most probably yes. Since IPv6 is deployed more on consumers side rather than enterprises. So it's not surprising that over the weekend IPv6 traffic hits a peak.
By default, most ISP routers firewall all IPv6 traffic (and IPv4 is implicitly firewalled via NAT). In an enterprise environment, dual stack support can get a lot more tricky and you need to ensure firewalls are property configured, or else regular workstations can be accessible via the public network.
Home connections have more IPv6 integration than businesses that have to configure their firewalls, etc.