@dpdrown Thanks again for the suggestion to use ´ethtool´ . The adapted Lua script now works. 
See LED color based on bandwidth usage for details.
Side note: ethtool -S creates different outputs depending on the interface. E.g. in_good_octets and out_octets do not exist for eth2. For eth2 those are called good_octets_received and good_octets_sent.
Any idea on the Realtime Graphs?