\documentclass[a4paper]{article}

\usepackage{pgfplots}

\pgfplotsset{compat=1.3}

\begin{document}

\begin{tikzpicture}
\begin{axis}[
	unit vector ratio=2 1,
	small]
    \addplot coordinates {(0,0) (1,1)};
    \addplot table[row sep=\\,col sep=&] {
        x & y \\
        0 & 1 \\
        1 & 0 \\
    };
\end{axis}
\end{tikzpicture}

\message{^^J^^JSECOND PICTURE}%

\begin{tikzpicture}
\begin{axis}[
	axis equal,
	small]
    \addplot coordinates {(0,0) (1,1)};
    \addplot table[row sep=\\,col sep=&] {
        x & y \\
        0 & 1 \\
        1 & 0 \\
    };
\end{axis}
\end{tikzpicture}
\end{document}

