Q-Q plot combined with Tukey mean-difference plot - re 'Visualizing Data by Cleveland (1993)

Dear All
I have had a very good look at the Graphics manual of Stata , but I have not been able to find any references to Tukey mean-difference plot in relation to Q-Q plots. The possibility of combining these two graphs are recommended by C leveland, and is also part of other packages - e.g. Statistix. Is there a reason for this?

Sincerely Søren Nielsen