How to plot only the tensor diagonals?

Hello guys,

I'm trying to perform a subplot where my V variable is not plotting. Highlighting that both variables P (1,1, :) and V (1,1, :) are tensors. Except that when putting this format accuses error. So I chose P (1,1) and V (1,1), because I'm only wanting the diagonals of all the matrices of the iterations. Can someone help me? I am sending the code and the figure you are plotting, but my P variable is not plotting.

figure(8)
subplot(3,1,1)
plot(t,P(1,1),'r', t,V(1,1),'*b')
title({'Estimates of errors'},'FontSize',16,'FontName', 'Times New Roman')
legend('P', 'V');
subplot(3,1,2)
title({'Estimates of errors'},'FontSize',16,'FontName', 'Times New Roman')
plot(t,P(2,2),'r', t,V(2,2),'*b')
legend('P', 'V');
title({'Estimates of errors'},'FontSize',16,'FontName', 'Times New Roman')
subplot(3,1,3)
plot(t,P(3,3),'r', t,V(3,3),'*b')
legend('P', 'V');
title({'Estimates of errors'},'FontSize',16,'FontName', 'Times New Roman')