Hey. Well it looks like a bug. Really you can only do the basic troubleshooting here.
1) Quit and reopen XCode
2) Restart machine (could be a very odd graphics glitch)
3) Try opening the file in a different window from XCode
4) Clear your XCode preferences by deleting/renaming ~/Library/Preferences/com.apple.Xcode.plist and reopen XCode (have it closed before you do this)
5) Report it as a bug if it is repeatable and nothing resolves the issue
Sorry, its just one of those things. Not really something you can do about it from your end.
can't visualize correctly core data model in xcode 4
Linear Mode
