Hi Ana,
Yes, either the plot command was intended for X or the ylabel is wrong. In the context of that example, I suspect they meant to have
plot(t,X)
But, having said that, the previous example on that page also plots S (without any labels), so maybe the mean to always plot the uncorrupted signal.
If you click on a star a the bottom of the page under "How useful was this information?" I'm pretty sure you'll get a pop-up box in which you can provide further feedback about that doc page.
