There are two issues with your code. First,
line([label_50 label_50], [round(N*0.5) round(N*0.5)])
is a point, not a line. So you probably won't be able to see it.
Second, since you are setting the x-axis scale to logarithmic, the left end of the line that starts from x=0
line([0 label_25], [n_25 n_25], 'Color','k')
will map to -inf and that line won't show up anymore on the plot. You can use a small x value instead to be able to see it. Something like:
line([0.1 label_25], [n_25 n_25], 'Color','k')