In reply to Paul Schmidt.

The primary reason is that the data in this article are distributed as Y~Exp(2), so the identity line is not relevant for this example. If I were looking at N(0,1) data, the identity line would be relevant.

A secondary reason is that the article shows how to overlay arbitrary lines of the form y = m*x + b. I did not feel the need to mention that the methods also work when m=1 and b=0.