How to plot a convergence graph for a double sum?

I'm trying to show converengence for a double sum but I can't figure out how to plot the convergence. I think I need to create an array to store the previous values of the series but I'm not sure how to do that.

This is the series I'm trying to graph:

This is my current code:

T=0; %terms used in series
q_m=0; 
M=15000; 
N=15000;
x=2.5;
y=3.5;
for n=1:N
    for m=1:M
        T=T+1;
        q_m1 = (400/(m*n*pi^2))*(cos(n*pi*4/10)-cos(n*pi*3/10))*(cos(m*pi*3/10)-cos(m*pi*2/10))*sin(m*pi*x/10)*sin(n*pi*y/10);
        q_m = q_m + q_m1;
    end
end
q_m
T

Answer 1

[VBBV]

T=0; %terms used in series
M=15; 
N=15;
q_m=zeros(M,1);
qq_m=zeros(M,1);
x=2.5;
y=3.5;
for n=1:N
  for m=1:M
      T=T+1;
      q_m1 = (400/(m*n*pi^2))*(cos(n*pi*4/10)-cos(n*pi*3/10))*(cos(m*pi*3/10)-cos(m*pi*2/10))*sin(m*pi*x/10)*sin(n*pi*y/10);
      q_m(m) =  q_m1;
  end;
qq_m(n) = sum(q_m);
end
plot(qq_m)

T

T = 225

You can modify the code as above to plot convergence

[VBBV]

Read also about symsum function to know more.

[Elizabeth McLean]

Thank you so much! This is just what I needed. I wasn't making the conection of using the index to fill in rows of a matrix.

[Star Strider]

@Elizabeth McLean — It is appropriate and appreciated to Accept answers that solve the problem.

[Elizabeth McLean]

Sorry about that, this is my first time asking a question. I've accepted the answer now.