Compares two values based on their values for this axis.
        
        :param      a | <variant>
                    b | <variant>

def compareValues(self, a, b):
        """
        Compares two values based on their values for this axis.
        
        :param      a | <variant>
                    b | <variant>
        """
        values = self.values()
        try:
            return cmp(values.index(a), values.index(b))
        except ValueError:
            return cmp(a, b)