You have a lot of 0s in your nc and x vector, hence you go back and forth to 0 position. You can see this if you use a data point in one of the lines and move along the data using keyboard arrow keys.
The reason is because your if statement is not always true, so you do not fill your nc and x vector in every run, so it fills it as default 0 for nc and you already prefilled x with 0s. You can either use an idx variable to find non zero entries as the code below, or change the way you store the values. I have no idea what you are trying to do so I am not sure if this is actually what you want.
%% EXAMPLE 1
n = 2; % n can only be a prime number
x = zeros(3,100); % initialize array with rows as cases for p
p = [80/100,85/100,88/100];
nc = x(1,:);
while (n < 100)
if isprime(n)
for i = 1:3
for j = n:-1:1
x(i,n) = (x(i,n) + j)^p(i);
nc(n) = n;
end
end
n = n+1;
else
n = n+1;
end
end
idx = nc~=0; % find non zero nc entries
plot(nc(idx),x(:,idx),'--o')
grid on
legend('Location','best')
xlabel('n')
ylabel('x_n')
title({'Lab1_A'})
