You plot() BEFORE you create the subplot; so the first plot is killed/destroyed after it is made when the subplot(2,2,1) call is made--then what is intended to be the second plot is plotted into that subplot axes. Same thing for the subsequent -- each is plotted into the subplot axes creathed ahead of it. There's no plot() command after the last suplot() axes is created; hence it's empty.
MORAL: Create the subplot() axis FIRST, then plot into it.
figure;
subplot(2,2,1)
plot(t,x11,'-*'),hold on;
plot(t,x(:,1),'-')
title('x1 vs Non-linear')
legend({'x1','Non-linear'},'Location','northeast')
txt = {'eq point [0.5,0.5]','initial point[0.51,0.51]'};
text(.4,10,txt)
subplot(2,2,2)
plot(t,x12,'-*'),hold on;
plot(t,x(:,2),'-')
...
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