The "network map" on the First Glasgow website sort of (very "sort of") had that, although for other operators it was very poor and it seems to have been taken down now. I've got a downloaded copy of it if you want me to post it.
The Lanarkshire Area map sort of has this too (still available, address below) but again it's very poor with showing other operators and it is out of date.
I'm working on a diagrammatic map to show what you describe but I'm only in the City area so far, I haven't made it as far as the suburbs yet.
I'm unsure if McGill's still have maps? I couldn't seem to find them on the website last time I checked.
I couldn't see any on the SPT site either.
The ones on the West Coast Motors site only show their own services, if I remember correctly.
Link to the maps on the First site:
Greater Glasgow - Network Maps Welcome to your First Bus Greater Glasgow network maps. Here, you can find Glasgow bus routes, in order to plan your journey across Glasgow more effectively and in advance. Whether you need to catch the number 9 to Glasgow city centre, or the 77 to the airport...
www.firstbus.co.uk