\documentclass[a4paper]{article}

\usepackage{pgfplots}

\pgfplotsset{compat=1.5}

\begin{document}

%\tracingcommands=2\tracingmacros=2
\begin{tikzpicture}
\begin{axis}[x=1cm,y=0.5cm,
	y dir=reverse,
]
\addplot expression[domain=0:3] {2*x};
\end{axis}
\end{tikzpicture}

\end{document}

