thanks for your reply, I carry experiment and find
ig_x + ig_y

and
z0[1] - z0[0]

to be equal to,
1.21376102688577

when integrating, with alpha from 0 to 1.
in the paper, they also have this figure,

does this mean, that instead of parameterization of a line,  we could parameterize a curve also, like a sigmoid curve, or a sinusoidal curve, or a circle and that would be a different attribution method.