'Return the separator line segments; helper for _add_solids.'

def _edges(self, X, Y):

    N = X.shape[0]
    if (self.orientation == u'vertical'):
        return [list(zip(X[i], Y[i])) for i in xrange(1, (N - 1))]
    else:
        return [list(zip(Y[i], X[i])) for i in xrange(1, (N - 1))]
