Try adding this to the end of theposted code:
text(xt(2:3:end), 188*ones(1,3), {'A-OSA','A-N','C-N'}, 'horiz','center', 'vert','top')
so the full code is now:
y = [316.45 292.14 319.96; 305.59 287.99 295.21;316.45 292.14 319.96; 305.59 287.99 295.21;...
316.45 292.14 319.96; 305.59 287.99 295.21;316.45 292.14 319.96; 305.59 287.99 295.21;305.59 287.99 295.21];
box on
hbar = bar(y);
set(gca,'xticklabel',{'Pre-test'; 'Post-test';'Pre-test'; 'Post-test'})
ylim([200 360])
ylabel('RT (ms)')
xticks([1 2 3 4 5 6 7 8 9])
xticklabels({'knn','svm','dt','knn','svm','dt','knn','svm','dt'});
xt = get(gca, 'XTick');
text(xt(2:3:end), 188*ones(1,3), {'A-OSA','A-N','C-N'}, 'horiz','center', 'vert','top')
Experiment with it to get the result you want.
