\documentclass{minimal}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
  \begin{axis}
    \addplot[color=red] coordinates {(1, 1) (2, 200)};
    \addlegendentry{plot 1 (\ref{pgfplots:plot2} xy added)}

    \addplot[color=blue, mark=x, forget plot] coordinates {(2, 200) (2.5, 100) (3, 50)};
    \label{pgfplots:plot2}

    \addplot[color=red, forget plot] coordinates {(3, 50) (4, 100)};
  \end{axis}
\end{tikzpicture}\\
Here the legend image \ref{pgfplots:plot2} is displayed correctly.
\end{document}

