Usually, I would assume this is a ping from your host's monitoring server to see if your server is online and responding. You can add logging to IPv6 this with ip6tables, for a look.
# LOGGING is a chain redirected from INPUT for testing
ip6tables -A LOGGING -j LOG --log-prefix="IPv6 Packet: "
If you wanted a detailed outline of what this traffic actually is, you can set up Wireshark or another packet-capturing tool on the server to find this out. Failing that, however, someone polling your server is hardly "nefarious". The level is actually unusually low.
It could also be TCP KeepAlives from your clients to the server. Do some research; there's not more we can say about this without log details.
EDIT: Actually, I noticed that all of the traffic on that graph is internal (private/priv). There's nothing wrong with it, and it's likely services communicating with each other, either on localhost or inside the computer's internal network.
It's quite likely those are IPv6
MULTICAST packets being sent every two hours