In reply to Quentin.

I think of it this way. If you look at the formulas for Tukey's pairwise comparison (Tukey-Kramer criterion), you see that is is a probability quantile divided by sqrt(2). Recall that sqrt(2) is the length of the diagonal of a square. The diffogram creates a scatter plot of the mean-mean pairs and equate the axes (to get a square plot), so that if you plot the confidence intervals diagonally, the geometry in the diffogram makes the sqrt(2) factor very natural. Other criteria may or may not have sqrt(2) in their formulas, but you can scale appropriately so that the geometry of the diffogram applies to those other tests as well. For a longer (and more rigorous) explanation, see Hsu and Peruggia (1991).