You're messing with the wrong thing -- it's xlim you want to make the same...
...
figure
hAx(1)=subplot(2,1,1); % save the handle for later when create the axes...
ylabel('NPV ($B)')
plot(x_value, Analyti /10^9,'--k');
hold on
p1 = plot(x_value, NPV_all'/10^9);
title('Impact of Discretizaion on NPV')
legend('Discrete NPV','continuous NPV', 'Analytical NPV')
xlabel('Discrete Time Method')
xticks([1:4])
xticklabels({'Years','Months','Days','Hours'})
ylim([8,11])
hAx(2)=subplot(2,1,2);
p2 = bar(period); hold on
xticks([1:4])
xticklabels({'Years','Months','Days','Hours'})
ylabel('Seconds')
xlabel('Discrete Time Method')
xlim(hAx(1),hAx(2).XLim) % put both plots on same x-axis range
You could make it somewhat easier if you were to use a categorical variable for x instead...
...
periods={'Years','Months','Days','Hours'};
x_value=categorical([1:numel(periods)],[1:numel(periods)],periods,'Ordinal',1);
figure
hAx(1)=subplot(2,1,1);
hL(1)=plot(x_value, Analyti /10^9,'--k'); hold on
hL(2)=plot(x_value, NPV_all'/10^9);
title('Impact of Discretizaion on NPV')
legend('Discrete NPV','continuous NPV', 'Analytical NPV')
xlabel('Discrete Time Method')
ylabel('NPV ($B)')
ylim([8,11])
hAx(2)=subplot(2,1,2);
hBar=bar(x_value,period);
ylabel('Seconds')
xlabel('Discrete Time Method')
