I and many of my colleagues have looked endlessly to find the text that this graph is associated with and so far there has not been any luck whatsoever. If anyone knows the citation affiliated with this diagram please let me know. Thank you all.
I have found the text associated with this graph. It is out of a United States military training text book: Training Circular TC 3-22.20 (FM 21-20) Army Physical Readiness Training August 2010 by United States Government US Army. The graph can be found in chapter 5 on page 5-4. The name for this graph is Figure 5-1. Soldier response/adaptation to overreaching, overtraining, and overuse.
Also, the Overtraining in Sport textbook is well worth a read if this is the sort of work you are interested in. Cheers.