Problem with plot in interval

I have a problem with this code:

When i give in input this:

fzer0(@(x)x.^2-4,[0 4])

Matlab give me a good plot.

When i give in input this:

fzer0(@(x)x.^2-4,[0 13])

Matlab give me a wrong plot, the output have not a plot!!

plot(fun(xo(1):0.00001:xo(2)), 'b - ', 'LineWidth', 2.5);
grid on;
yline(0, '--r', 'y = 0');
xlabel('x');

str=func2str(fun); %stringa della funzione
newstr=erase(str(5:end), '.'); %cancella dall'espressione il function handle e cancella il punto
tit=strcat('f = ', newstr); 
title(tit);

text(double(m), double(0), strcat( 'x = ', num2str(m) ) );

set(gcf, 'color', 'g');
set(gca, 'FontSize', 9);

[Walter Roberson]

I am not clear what fun is here?

Also we have no information about xo(1) and xo(2) ?

[valerio auricchio]

Fun is a function that in the 2 case is x^2-4.

this is the code for xo(1) and xo(2):

while(abs(diff(xo))>= p.Results.TOL*max(abs(xo)) && abs(fun(m))>TOLF && nit<p.Results.NMAX)
    if(fun(m)*fun(xo(1))<0)
        xo(2)= m;
    else
        xo(1)= m;
    end
    nit=nit+1;
    m = xo(1)+0.5*diff(xo);
end
if(abs(diff(xo))> p.Results.TOL*max(abs(xo)) && abs(fun(m))>TOLF && nit==p.Results.NMAX)
    warning("limit reached... calculation stop! ");
end
x=m;
output.fx = fun(x);
output.niter = nit;

[Walter Roberson]

Please put everything together in a form that we can run, in order to test your code.