You mean like
xlim([100 200]);
That will set up the "viewport" of your plot. Maybe you're plotting stuff from 0 to 1000 but only points with x values of 100-200 will be seen in your plot and points outside there will be not visible.
Or do you want to keep the x data but just reassign the x tick labels? Let's say you have 300 time points as your x, and voltage as your y. But let's say that element 1 was at time 100 milliseconds, and element 300 represents 200 milliseconds. Well you can continue to plot all 300 data points and just stick new labels onto the tick marks. Following the example from the help:
set(gca,'XTickLabel',{'100 ms';'150 ms';'200 ms'});
to relabel 3 tick marks.
