Great post, thanks. Agree, the diffogram is very cool. I was going to ask if there was an intuitive explanation for the length of the reference lines, but after reading a bit of Robin High's paper, I'm going to guess the answer is "no"? : ) "A third axis (implied on the graph but not printed by SAS ODS Graphics) exists from the top left to the lower right corners of the plotting area. This hidden axis represents the magnitude of the differences of the LSMEANS defined on the horizontal and vertical axes scaled in such a way [i.e., divided by SQRT(2)] that the confidence interval for the difference crosses the line of equality when the interval contains 0. It also provides values for the endpoints of the confidence intervals necessary for equivalence testing."