Our server for handling mobile data and direct L2TP connections and a few other minor things is showing issues with the usage graphs. We are investigating and may reload that server a few times with extra debug. The reload takes a fraction of a second but means reconnecting any in progress mobile data or external L2TP which can take a few seconds when it happens. Sorry for any inconvenience but we have not been able to reproduce the problem on test systems.
We believe we have got to ghe bottom of the issue with graphs.