例えばこんな感じでforループの前に設定を入れるのではどうでしょうか？
theta = linspace(-pi,pi,500);
xc = cos(theta);
yc = -sin(theta);
plot(xc,yc);
axis equal
%%
xt = [-1 0 1 -1];
yt = [0 0 0 0];
hold on
t = area(xt,yt); % initial flat triangle
% title, axis, legend settings
title('hello')
xlim([-2 2])
ylim([-2 2])
legend({'Line', 'Area'})
hold off
for j = 1:length(theta)-10
xt(2) = xc(j); % determine new vertex value
yt(2) = yc(j);
t.XData = xt; % update data properties
t.YData = yt;
drawnow limitrate % display updates
end
