Frist, decide on how many ticks labels you want, Then, create a cell array with the labels. Set the axis ticks and tick labels accordingly. The script below shows the general idea. There are 200 data points and 9 tick labels, equally spaced.
y = rand(1,200);
plot(y);
ax = gca;
ax.XTick = 0:25:200;
labels = { 'a', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j' };
ax.XTickLabels = labels;
