I tried to reporduce the issue using fake data since I don't have access to PS1... files. I also notcied your reproduction code doesn't include generating a legeng (lgd is not defined). After replacing your data with fake data and adding a call to legend, this is the result below.
figure;
yyaxis left; % Use the left y-axis
plot(1:5, 'DisplayName', 'Scenario 1', 'Color', rand(1,3));
hold on;
plot(1:5, 'DisplayName', 'Scenario 2', 'Color', rand(1,3));
plot(1:5, 'DisplayName', 'Scenario 3', 'Color', rand(1,3));
plot(1:5, 'DisplayName', 'Scenario 4', 'Color', rand(1,3));
ylabel('Optimal (h)');
% Adding the second column of D with a second y-axis
yyaxis right; % Use the right y-axis
plot(1:5, 'DisplayName', 'Scenario 2', 'Color', rand(1,3));
ylabel('Modules');
% Adding legend
ldg = legend();
lgd.Position = [0.85, 0.65, 0.1, 0.1];
% Adding x-axis label
xlabel('Demand');
% Display the grid
grid on;
