Does this mean we'll have separate results for train and plane ? As well as a third result that is the mix of the two? Multiple plots, and lists of cities ?

I think you should compute the mean of the plane, and add the train, and provide the sum. I don't think we should provide other plots. What is more important I believe is to provide a list of minimum distance for flying. Some scientists are flying only for distance larger than 1000 km now. I would do 300, 500, 700 and 1000 km in a menu.

Ok, this is done ! 23g per km, as discussed.