\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[legend image post style={mark=*}, legend entries={Correct}]
\addplot [mark=none,smooth,domain=0:2]
{-x*(x-2)}; \label{myplot}
\addplot [only marks,forget plot]
coordinates {(0.5,0.75) (1,1) (1.5,0.75)};
\end{axis}
\end{tikzpicture}

\ref{myplot} Incorrect
\end{document}
