Consider using the interp1 function with the 'nearest' method on your data, then plot the result produced by interp1 as a function of the interpolation vector you used. Experiment with it to get the result you want. It should give you the ‘stairs’ effect.
Otherwise, you would have to hack into the code for the plotyn version you are using, and substitute stairs for plot. I don’t suggest modifying someone else’s code unless you thoroughly understand how it works.