\documentclass[a4paper]{article}
\usepackage{german}
\usepackage[utf8]{inputenc}

\usepackage{pgfplots}

\usepackage{pgfplotstable}
\usepackage{booktabs}
\usepackage{array}
\usepackage{colortbl}

\begin{document}
\begin{tikzpicture}
	\begin{axis}[stack plots=y,domain=0:5]
	\addplot (x,x);
	\addplot (x,x);
	\legend{1,2}
	\end{axis}
\end{tikzpicture}

\begin{tikzpicture}
%\tracingcommands=2\tracingmacros=2
	\begin{axis}[xmode=log,stack plots=y,domain=0:5]
	\addplot ({exp(x)},x);
	\addplot ({exp(x)},x);
	\legend{1,2}
	\end{axis}
\end{tikzpicture}

\begin{tikzpicture}
%\tracingcommands=2\tracingmacros=2
	\begin{axis}[ymode=log,stack plots=x,domain=0:5]
	\addplot {exp(x)};
	\addplot {exp(x)};
	\legend{1,2}
	\end{axis}
\end{tikzpicture}
\end{document}

