[Haskell-cafe] Finding the Convex Hull (Problem 12 of Real World Haskell)

I wrote a "solution" to this problem, but it appears to return incorrect results. There's a pastebin of the code at http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2121 and a picture of the inputs, outputs, and expected results graphed at http://img510.imageshack.us/img510/9971/resultsg.jpg I'm wondering if this is a flaw in my code, my understanding of the problem, or both. Any ideas on how to track this one down would be very much appreciated. Thank you! -- ヽ(^o^)ノ -rob