[Tex/LaTex] Draw a line between data points in two different pgfplots (subfigures)

Question

I have two different pgfplots in two subfigures. I want to draw a line between the red circled data points and the blue ones. I added lines manually with LibreOffice Draw to show you what I mean.

My code is the follwing:

\begin{figure}[!htbp]
\centering
    \subfigure[Ausgangszustand]{
    \resizebox{0.45\textwidth}{!}{
    \begin{tikzpicture}
    \begin{axis}[
        xlabel={Zeit [d]},
        ylabel={Auslastung [\%]},
        axis lines=left,
        xmin=0,
        xmax=7.5,
        xtick={1,...,8},
        ymin=0,
        ymax=105,
        thick,
        grid=both
        ]
    \addplot[smooth,thick] plot coordinates {
        (0,35)
        (1,60)
        (2,20)
        (3,95)
        (4,45)
        (5,30)
        (6,60)
        (7,55)
    };
    \addplot+[only marks,mark=o,mark options={scale=3},text mark as node=true] coordinates {
            (3,95)
    };
    \addplot+[only marks,mark=o,mark options={scale=3},text mark as node=true,color=blue] coordinates {
            (2,20)
    };
    \end{axis}
\end{tikzpicture}
}}
\qquad
\subfigure[Zielzustand]{
\resizebox{0.45\textwidth}{!}{
\begin{tikzpicture}
    \begin{axis}[
        xlabel={Zeit [d]},
        ylabel={Auslastung [\%]},
        axis lines=left,
        xmin=0,
        xmax=7.5,
        xtick={1,...,8},
        ymin=0,
        ymax=105,
            thick,
            grid=both
            ]
        \addplot[smooth,thick] plot coordinates {
            (0,45)
            (1,55)
            (2,40)
            (3,70)
            (4,55)
            (5,35)
            (6,50)
            (7,45)
        };
        \addplot+[only marks,mark=o,mark options={scale=3},text mark as node=true] coordinates {
                (3,70)
        };
        \addplot+[only marks,mark=o,mark options={scale=3},text mark as node=true,color=blue] coordinates {
                    (2,40)
        };      
        \end{axis}
    \end{tikzpicture}
    }}
    \caption{Glättung von Lastspitzen}
    \label{fig:Lastspitze}
\end{figure}

I want to draw the lines beginning and ending in the exact data points in the graphs. I did some googling but couldn't find an answer.

Answer 1

You could use TikZ overlay functionality, but then you will have to refrain from using \resizebox because it is messing with the markers set to the .aux-file.

\documentclass{article}
\pagestyle{empty}% for cropping
\usepackage{pgfplots,subfigure}
\begin{document}
\begin{figure}[!htbp]
    \centering
    \subfigure[Ausgangszustand]{
        \begin{tikzpicture}[remember picture]
            \begin{axis}[
                    xlabel={Zeit [d]},
                    ylabel={Auslastung [\%]},
                    axis lines=left,
                    xmin=0,
                    xmax=7.5,
                    xtick={1,...,8},
                    ymin=0,
                    ymax=105,
                    thick,
                    grid=both,
                    width=0.45\textwidth
                ]
                \addplot[smooth,thick] plot coordinates {
                    (0,35)
                    (1,60)
                    (2,20)
                    (3,95)
                    (4,45)
                    (5,30)
                    (6,60)
                    (7,55)
                };
                \node[red,draw,circle,inner sep=4pt] (red-1) at (axis cs:3,95) {};
                \node[blue,draw,circle,inner sep=4pt] (blue-1) at (axis cs:2,20) {};
            \end{axis}
        \end{tikzpicture}
    }%
    \qquad
    \subfigure[Zielzustand]{
        \begin{tikzpicture}[remember picture]
            \begin{axis}[
                    xlabel={Zeit [d]},
                    ylabel={Auslastung [\%]},
                    axis lines=left,
                    xmin=0,
                    xmax=7.5,
                    xtick={1,...,8},
                    ymin=0,
                    ymax=105,
                    thick,
                    grid=both,
                    width=0.45\textwidth
                ]
                \addplot[smooth,thick] plot coordinates {
                    (0,45)
                    (1,55)
                    (2,40)
                    (3,70)
                    (4,55)
                    (5,35)
                    (6,50)
                    (7,45)
                };
                \node[red,draw,circle,inner sep=4pt] (red-2) at (axis cs:3,70) {};
                \node[blue,draw,circle,inner sep=4pt] (blue-2) at (axis cs:2,40) {};
            \end{axis}
        \end{tikzpicture}
    }%
    \begin{tikzpicture}[remember picture,overlay]
        \draw[dashed,red] (red-1) -- (red-2);
        \draw[dashed,blue] (blue-1) -- (blue-2);
    \end{tikzpicture}
    \caption{Gl\"attung von Lastspitzen}
    \label{fig:Lastspitze}
\end{figure}
\end{document}