How to plot iterations?

Hi I have this simple code and I would like to make graph between rez_p and iterations. I have created this, but the plot, that is coming out is blank. Could you please tell me, what I am doing wrong? Thank you so much.

nx=80;
nx=40;
nit=30000;
for it=1:nit
    A=Pn-P; %Pn and P is matrix 81x41.. I dont have it here
    sou=0.0;
    for i=1:nx+1
        for j=1:ny+1
            sou=sou+A(i,j)*A(i,j);
        end
    end
    rez_p=sqrt(sou)/(nx*ny);
  
    if mod(it,30000)==0 || it ==1; %plot
    figure
    plot(it, rez_p)
    end   
end

[KALYAN ACHARJYA]

I have created this, but the plot, that is coming out is blank

Because rez_p return as single value,not range of values. See the @Star answered, he cretes a rez_p(it) with equal length of it.

for ...
end
it=1:nit;
plot(it,rez_p)