\documentclass{minimal}
    \usepackage{pgfplots}
        \pgfplotsset{compat=newest}
\begin{document}
    \begin{tikzpicture}
        \begin{axis}
            \begin{scope}[only marks,mark=*]
                \addplot[red] coordinates { (0,0) (1,1) };
            \end{scope}
            \begin{scope}[/pgfplots/every axis plot post/.style={mark=none}]
                \addplot coordinates { (0,1) (1,0) };
            \end{scope}
                \addplot coordinates { (0.5,1) (0.5,0) };
        \end{axis}
    \end{tikzpicture}
\end{document}

