I switched to kernel 3.14.1-trusty, and the problem is fixed. Credit goes to this AskUbuntu question:
http://askubuntu.com/questions/409007/why-doesnt-system-monitor-show- activity-over-ethernet-connections -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1241894 Title: Network History no longer works in System Monitor since 13.10 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/gnome-system-monitor/+bug/1241894/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
