\begin{tikzpicture}
  \begin{axis}[
    width=.45\textwidth,
    smooth,
    stack plots=y,
    area style,
    enlarge x limits=false]
  \addplot coordinates
    {(0,1) (1,1) (2,2) (3,2)}
    \closedcycle;
  \addplot coordinates
    {(0,1) (1,1) (2,2) (3,2)}
    \closedcycle;
  \addplot coordinates
    {(0,1) (1,1) (2,2) (3,2)}
    \closedcycle;
  \end{axis}
\end{tikzpicture}
