sortByCoTan :: [Point] -> [Point]
sortByCoTan [] = []
sortByCoTan [a] = [a]
sortByCoTan (a:as) = a : sortByFirst (sortByCoTan as) where
    sortByFirst [b] = [b]
    sortByFirst (b:c:cs)
        | (myCoTan a b)>= (myCoTan ac) =  b : sortByFirst (c:cs)
        | otherwise                    =  c : sortByFirst (b:cs)
        where
            myCoTan :: Point ->  Point ->  Double
            myCoTan p1 p2 = (pointXval p2 - pointXval p1) /
                (pointYval p2 - pointYval p1)