The Reputation history of my Super User account is showing a lower upper limit on the graph compared to my current reputation. I think this is wrong, is it?
It's probably off by 100 - the amount you get for associating your accounts.
If you look at my Server Fault profile you'll see that the graph stops at just over 600 (as of this writing) while my rep is 731.
The discrepancy is the 100 point bonus I got for associating my accounts.