This is an essential need for my application, and would be very helpful for those with laptops that regularly switch between wired and wireless connections.  

I have 40 machines which make up a mobile Internet Den setup which is used at various locations such as emergency shelters throughout the year.  A random number of the machines may be connected either by wired ethernet or wireless to the system router.  

It would therefore be *VERY* handy if there was an AUTO choice for the network interface to be monitored.  

This conceivably would be just a matter of examining the current kernel routing table (route) and looking for the interface device used as the default gateway and monitoring that.  

This feature would also eliminate the need for manual re-configuration every time the interface changes.  Currently, EVERY machine in the setup must be checked and set for proper interface, and since the machines use XFCE Kiosk mode to lock the panel from users, it all becomes a real pain.

It would be great also if the displayed text would either: 

1) allow at least 2 Text choices depending on the interface type in use (e.g. if ethernet (ethX), display 'Net' and if wireless (wlanX), display 'Link') or
 
2) allow the system name of the interface (wlan2, eth3, etc) being monitored to display.   Having the display text change gives immediate visual feedback on the interface in use/monitored.

Of course, the current manual settings would need to remain available for those wanting to run multiple monitors on specific interfaces.

Please consider adding this feature...hopefully, soon!