\begin{tikzpicture}
  \begin{axis}[
    width=.45\textwidth,
    legend pos=north west,
    xlabel=$x$,
    ylabel=$y$
  ]
  \addplot {x^3};
  \addplot[ybar,fill=red,draw=red!60,
    ybar legend,mark=none,samples=5]
    {-30*(x+4)};
  \legend{first,second}
  \end{axis}
\end{tikzpicture}
