Perhaps this —
x1 = [1 1 1 2.5 5];
y1 = [5 2.5 1 1 1];
x2 = [2 2 5];
y2 = [5 2 2];
figure
hold on
plot(x1, y1, LineStyle=":");
plot(x2, y2, LineStyle=":");
xlim([0, 6])
ylim([0, 6])
patch([x1 flip(x2)], [y1 flip(y2)], [1 1 1]*0.5, 'EdgeColor','none')
% fill([x1, x2], [y1, y2], 'k', 'FaceAlpha',0.2)
hold off
The patch function fills a closed region, defined by its arguments. If you want to fill between the lines, that is relatively straightforward.
.
