In the print composer (also in the layer selector) I can choose to automatically display the amount of objects in that category in square brackets. That is useful for point features/layers. But in the case of line layers I would like to show the total length of all lines in that category (sum) in just the same fashion.
It is possible to get that info using the GroupStats plugin. But I am looking for a way to automatically keep my legend updated with that info. Just as neat and uncomplicated as with the square brackets for the total count of objects.