Question: separately plot each line

how is possible to draw each line separately?

n1 := 1; n2 := 1.45; n3 := 3.5;

z0 := 120*Pi; d1 := 1; d2 := 1; d3 := 1;a := 1;

k0 := 2*Pi/lambda;

ub := sqrt(n1^2-`βb`^2); vb := sqrt(n2^2-`βb`^2); wb := sqrt(n3^2-`βb`^2);

Ub := k0*ub*d1; Vb := k0*vb*d2; Wb := k0*wb*d3;

z1 := z0/ub; z1b := z1/z0;

a0 := -tan(Vb)/(wb*ub)-tan(Ub)/(vb*wb)-tan(Wb)/(vb*ub)+tan(Ub)*tan(Vb)*tan(Wb)/wb^2;

b0 := tan(Ub)*tan(Vb)/(vb^2*wb)+tan(Vb)*tan(Wb)/(vb^2*ub)+tan(Ub)*tan(Wb)/(vb*wb^2)-tan(Vb)*tan(Ub)/(wb*ub^2)-tan(Vb)*tan(Wb)/(wb^2*ub)-tan(Wb)*tan(Ub)/(vb*ub^2);

c0 := -tan(Ub)/(vb*wb*ub^2)-tan(Wb)/(vb*wb^2*ub)+tan(Vb)*tan(Wb)*tan(Ub)/(vb^2*ub^2)-tan(Vb)/(vb^2*wb*ub);

U0 := k0*ub*a; m := 0;

y := a0*z1b^2+c0+(a0*z1b^2-c0)*cos(2*U0+m*Pi)+b0*z1b*sin(2*U0+m*Pi)

with(plots, implicitplot);

implicitplot(Re(y), lambda = 1 .. 2, `βb` = .5 .. 3.5, gridrefine = 5)