In section 7.4 (pages 78-79), I have you set up an NSArrayController for the DataPoint array in the document's model object. NSArrayController needs to know the name of the class of the objects in its array (in this case, DataPoint).  
 
Section 7.4 does not tell you to set the Object Class Name, in the inspector (command-1), to DataPoint. You have to do this for archiving and dearchiving the array in a document to work properly. The sample code accompanying Chapter 7 (and later) does set the class name. 
 
Thanks to Bruce Truax for diagnosing the problem and identifying the fix.