Hello again.
Assuming the assumption was correct in the first place , it may be that what the graph is trying to show is meant to be general . The Vmu line does stop suddenly at some point , it may be the flat rating cut-out point or low temp sea level conditions.
Now with regard with the comments of the last poster , i am not going to argue that knowledge of this or anything of this depth will necessarily make someone a better pilot , but i did find throught the years that whetever you know comes back as something usefull one way or another. But even more so , there are just some things that you feel like knowing in depth for it to make sence . If i read anywhere that a particular speed as CAS /IAS varies with density , then it does give me a definate reason to find out how .
Regards
stickN'Rudder007