co2 (kg) distance (km) plane trips train trips

should be renamed to: co2_kg, distance_km, plane_trips, train_trips.

Thanks !

We can remove the spaces if they spell trouble, but, are the parentheses problematic ? We use them to parse the units, and without them trips would become a unit.

Yes the parenthesis are problematic. If you want you can remain plane_trip_number, train_trip_number: that would work well for me. I am reading the column names in Datagraph and to manipulate the column for instance dividing by 1000 to get co2_kg in tons, Datagraph will not work if you name co2 (kg). It doesn't like space and ( ).