Here I looked at the possibility of using xfce4-netload-plugin. It works for monitoring network speed. However, due to Qubes OS’s specifics, I need to start it in a Panel from sys-net which is not quite clean visually - the panel runs in its own small window with a title bar, floating etc.
For measuring used traffic on my Linux systems I usevnstat. However, it stores its db to `/var’ which means it will “forget” its data when sys-net is restarted. I am still looking for a proper way to fix this or for an alternative. If someone has the answer, this would be good completion to the current thread.