Well I can't see any graph, but it's probably a linear growth with the function f(x)= 0x+2 <=>
f(x) = 2
A linear growth is based on f(x)=ax+b, and since there isn't any growth in cats, a must be 0. x*0 = 0, therefore it doesn't matter if he stayed for 4 months or a 100 years, if there was the same amount of cats. B = 2, since that's how many cats he started out with.
