% pgf v2.1, pgfplots v1.4.1
\documentclass{minimal}
    \usepackage{pgfplots}
        \pgfplotsset{compat=newest}
\begin{document}
    \begin{tikzpicture}
        \begin{axis}
            \addplot coordinates                    { (0,0.25) (1,0.25) };
            \addplot [black] coordinates            { (0,0.5) (1,0.5) };
			% no explicit 'mark=*':
            \addplot [only marks] coordinates       { (0,0) (1,1) };
            \addplot [only marks,red] coordinates   { (0,1) (1,0) };
			\legend{a,b,c,d}
        \end{axis}
    \end{tikzpicture}
\end{document}

