Here are two options with slightly different outcomes.
Option 1
Modify the upper line only
Y = [1, 5, 3;
3, 2, 7;
1, 5, 3;
2, 6, 1];
figure()
area(Y)
hold on
plot(sum(Y,2),'Linewidth', 2, 'LineStyle', '-.')
Option 2
Modify the line that surrounds the entire outline of the upper area object.
Y = [1, 5, 3;
3, 2, 7;
1, 5, 3;
2, 6, 1];
figure()
h = area(Y); % get the area object handles
set(h(3), 'Linewidth', 2, 'LineStyle', '-.')
