There's no way the code posted will do what you say; it has fatal syntax error both lines--in the first the 'XTick' vector must be specified as (2:2:8) as it is required for tick values to be increasing when set irregardless of the sense of the direction named parameter; it does the reversal internally. As written (8:2:2) it's a silent error but turns the ticks off because with a positive increment but the end value less than start, it returns an empty array.
plot(set,'-ys')
fails because there's no argument to the set function.
As to the original question, you'll have to interpolate or otherwise find and plot values for the other X locations to place a square there or it would be possible with some additional effort to use text or annotation to add the markers where wanted independently of the plotted data.
