For each piecewise-defined function,
(a) calculate $f(-1), f(0), f(1)$, and $f(2)$, and (b) sketch a graph of $f$.
$$
f(x)=\left\{\begin{aligned}
3 x+1, & \text { if } x \leq 0 \\
4, & \text { if } 0<x \leq 1 \\
x^{3}, & \text { if } x>1
\end{aligned}\right.
$$