You may use function text to insert text on the figure.
You may define the text position better by assigning the position manually.
Edit for correction of the offset line:
Use 0.1% mentioned in the text inside your code instead of 6% which may not be practical. Anyway, you may adjust on the following code
plot(E,S);
grid on
hold on
xlabel('Strain')
ylabel('Stress')
title('Stress-Strain Diagram')
textname = {'A','B','C','D'};
pointA = 65000;
pointB = 68897.88;
pointC = 68897.88;
pointD = 112832.76;
pointAll = [pointA,pointB,pointC,pointD];
for k = 1:4
xpos = E(S<pointAll(k));
if k<3
offset = -0.002;
else
offset = 0.002;
end
text(xpos(end)+offset,pointAll(k),textname{k},'FontSize',14,'FontAngle','italic');
end
%% Proportional Region (Region yield stress at 0.1% offset)
%
%
idx.proportional = sum(S<pointA); % Index for proportional region
slope = (S(idx.proportional)-S(1))/(E(idx.proportional)-E(1)); % Calculate slope
new_E = E + 0.001; % Adding offset for strain
new_S = slope*E; % New stress for same slope
idx.plot = new_S<pointD; % Limit the plot for stress below maximum stress
plot(new_E(idx.plot),new_S(idx.plot),'r--'); % Plot the offset line
hold off
