The graph showing number of decayed nuclei verses time t is :

$N = N_0 e^{-\lambda t}$

$\text{Number of decayed nuclei} = N_0 - N = N_0(1 - e^{-\lambda t})$

$t=0 \Rightarrow \text{decayed nuclei} = 0$

$t \to \infty \Rightarrow \text{decayed nuclei} \to N_0$

$\text{Thus, graph starts from } 0 \text{ and increases asymptotically to } N_0$

The graph is an increasing exponential curve approaching $N_0$ asymptotically.