The manual states:
Toggle on Traffic Monitor, select the desired interface from the dropdown list, and then you can see the packets received and sent via this interface in the past ten minutes. This function is disabled by default.
I don't think that description is correct for the graph in that it's not measuring the number of packets. The scale is in MB which is a meaningless scale for packets. However, I've calculated that it's most likely megabytes/second. The underlying link is been measured at ~25,000Mbps which does correspond to ~2.5MB/s.
So the graph is showing MB/s - and that my client's link is totally saturated by people streaming :-) But that's a different problem to solve.