the way that you are using stem function, the x axis would be the index number.
So your xFFT is an array of 201 double precision numbers. the x axis is from 1 to 201.
So pretty much you can interpret it as sample number.
use this form:
stem(xValues, YValues)
if you want to show something else as your x-axis.
Two more thing (not in your question)
1) usually use fftshift before plotting your stuff.
2) You have two calls to stem() function. The second one will override the first one. So either use 'hold on' to have both graph in one figure or if you want them in two separate figure then change it to something like this:
figure, stem(xFFT);
figure, stem(x_psd);
