There are plot3, contour3 versions of plot, contour for which you would have to add a third coordinate for each 2D plot, if that is what you are trying to do. For instance
plot(x1, y1)
plot(x2, y2)
plot3(x1, y1, repmat(z1, size(x1)))
hold on
plot3(x2, y2, repmat(z2, size(x2)))
