@classmethod
def FromXYPairs(cls, a, b):
    """
        Construct a linear relationship from two (x,y) pairs
        """
    l = cls()
    l.p1 = (b[1] - a[1]) / (b[0] - a[0])
    l.p0 = a[1] - l.p1 * a[0]
    return l
