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