\documentclass[a4paper]{article}

\usepackage{pgfplots}

\pgfplotsset{compat=1.3}

\begin{document}
\vrule height 2cm
\begin{tikzpicture}
\begin{axis}
	\addplot coordinates 
		{(-2,0) (-1,1) (0,0) (1,1) (2,0)};
\end{axis}
\end{tikzpicture}
\begin{tikzpicture}
\begin{axis}[y=2cm]
	\addplot coordinates 
		{(-2,0) (-1,1) (0,0) (1,1) (2,0)};
\end{axis}
\end{tikzpicture}

\end{document}

