\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.12}

\begin{document}
\begin{tikzpicture}
\begin{axis} [ymax=11]
\addplot+ [no marks] coordinates {(0,0) (1,0) (1,1)};
\draw [<->, dashed] (0,0.5) --  +(0:1);
\draw [<->, dashed] (0.8,0) --  +(90:1);
\end{axis}
\end{tikzpicture}
\end{document}
