That'll be your router settings. There'll be something in the settings about blocking ping requests, turn that off and the graph will start working correctly.
There was indeed a tick box with no tick in it in the settings - "Respond to ICMP echo requests sent to WAN IP". Stuck a tick in it now, I'll monitor it for a while. Hopefully the lag spikes are just a temporary glitch on my line, as it's been rock solid for the last few months. Cheers
