JWaddell, thank you very much once again for posting a solution and I apologize for taking so long to get back to this topic.

The sequence of R commands that you gave work very well.

However, what I (and many others in the science, engineering, and data-collection communities) require is to have the actual measured values (X1, X2, X3 in your example) be presented on a linear scale on the X axis (not the quantiles).

When I add datax=TRUE to the first qqnorm comand, and/or in the call or the body of the function addQQpoints, everything goes bad… I played with this for a long time and gave up…

Can you please help?