Yes. I think if we had those kinds of numbers we'd have a better idea of the whole story. The only way I could imagine getting anything like that though is if someone had an in with the Diamond/Alliance numbers (and even then only if Alliance still had the numbers post-merger since IIRC they were formed after Wizards acquired TSR).An interesting point of comparison would be a chart on RPG sales in general. it might give some insight into whether it was Vampire biting into D&D sales, or if all RPGs were down effectively equally because of CCGs or other factors.
Anecdotally when TSR went under in '97 the two local game store owners I would chat with were sure that was the end of RPGs in general because D&D was back to being their biggest RPG seller after Vampire had taken off for a while and then settled back down. They were making their money mostly off of CCGs (mostly Magic), with one of the stores also apparently making more off of Warhammer than RPGs (though Magic was his big cash cow). Local anecdote is never a good substitute for data, so I'd really like to see the numbers.