The diagrams are different, because the scaling differs. This is exactly what is expected, when you define the x component of the data or omit this information. Therefore it is not clear, what your question is. You draw two different things, then you can expected, that they look different. To get the same output, you can equalize the scaling and limits:
subplot(1,2,1);
plot(z)
axis tight
subplot(1,2,2);
plot(t,z)
axis tight
