Question: Procedure Plot return value


i have a Problem with maple. I have written this simple procedure

local i,l;
for i from 1 to floor(r) do
    if isprime(i) and (i mod 4 = 1) then
    end if;
end do;
return l;
end proc:    


When i use it like this 

k := myPi_1(80);

it works

But when i triy to plot it

plot(myPi_1(t), t = 0 .. 500, numpoints = 500, thickness = 2, color = black);

i get this message
Warning, expecting only range variable t in expression myPi_1(t) to be plotted but found name myPi_1

And no plot

Thank you for any Help





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