Not from Around Here pointed me in the direction of an excellent post on depression and culture shock on An American Bedu's blog. The post itself is interesting, the comments even more so. Culture shock is so much more complicated than many people realize. Many people think it goes away if you live long enough in the 'foreign' culture, but that's not always true. Some people never get over it and choose to go back home, as expat21 discussed recently on her blog. However, it's not true that returning 'home' will always be an improvement. There is truth in the saying "You can never go home" and reverse culture shock can be just as powerful as the regular kind!
Most of the 'graphs' of culture shock I've found online are based on the foreign-exchange student/temporary stay abroad experience rather than that of the immigrant: