How to name a spypoint in a tikzfigure to access it via node name

Question

I want to access two spypoints in a tikzpicture environment via node name. Unfortunately, naming the spypoint doesn't seem to work in the same way as it does for the magnifyglass node.

Here is my MWE:

\documentclass{standalone}

\usepackage{pgfplots}
\usetikzlibrary{spy}

\begin{document}

\begin{tikzpicture}

        \begin{scope}[spy using outlines={rectangle, magnification=4, connect spies}]

        \begin{axis}[%
            height=5cm,%
            width=10cm, %
            ymin=-1, ymax=21, %
            xmin=0, xmax=1000, %
            xticklabels={},%
            ytick={0, 10, 20}, %
            name=first,%
            ]

            \coordinate (spypoint1) at (axis cs:30,10);
            \coordinate (magnifyglass1) at (axis cs:300,3.3);

            \coordinate (spypoint2) at (axis cs:850,3.3);
            \coordinate (magnifyglass2) at (axis cs:650,3.4);

        \end{axis}

        \begin{axis}[%
            height=4cm,%
            width=10cm, %
            ymin=-1, ymax=1, %
            xmin=0, xmax=1000, %
                at={(first.below south west)}, yshift=-0.1cm,  anchor=north west, %
            ]

            \coordinate (spypoint3) at (axis cs:30,0);
            \coordinate (magnifyglass3) at (axis cs:300,0.4);

            \coordinate (spypoint4) at (axis cs:850,-0.1);
            \coordinate (magnifyglass4) at (axis cs:600,-0.4);

        \end{axis}

        \spy [black, height=12mm, width=20mm, name=a1] on (spypoint1)
            in node [fill=white,name=a] at (magnifyglass1);

        \spy [black, height=12mm, width=22mm] on (spypoint2)
            in node [fill=white,name=b] at (magnifyglass2);

        \spy [black, height=12mm, width=20mm, name=c1] on (spypoint3)
            in node [fill=white,name=c] at (magnifyglass3);

        \spy [black, height=12mm, width=22mm] on (spypoint4)
            in node [fill=white,name=d] at (magnifyglass4);

        \end{scope}

        \draw[dashed] (a) -- (b);
        \draw[dashed] (c) -- (d);

        \draw[red] (a1) -- (c1);

    \end{tikzpicture}

\end{document}

As you can see, the connections for \draw (a) -- (b) and (c) -- (d) work fine, while the connection (a1) -- (c1) doesn't work. I get the error message

No shape named a1 is known.

and the same for (c1).

Without the \draw[red] (a1) -- (c1); line, the picture looks as follows. The red line resembles what my expected result of adding the line would be

Can anyone tell me the correct way to name the spypoint node? I didn't find anything in the pgfplots or tikz manual. Thank you in advance.

Answer 1

Elaborating on Stefan Pinnow's comment, and using this answer: How can I change the line width of the spy-on node in tikz spy library?, here is how a spy on node can be named (it's quite counterintuitive to modify an every style locally for one node but it's what Tantau suggests in the manual).

        \spy [black, height=12mm, width=20mm, every spy on node/.append style={name=a1}] on (spypoint1)
            in node [fill=white,name=a] at (magnifyglass1);

The complete code:

\documentclass{standalone}

\usepackage{pgfplots}
\usetikzlibrary{spy}

\begin{document}

\begin{tikzpicture}[every spy on node/.style={name}]

        \begin{scope}[spy using outlines={rectangle, magnification=4, connect spies}]

        \begin{axis}[%
            height=5cm,%
            width=10cm, %
            ymin=-1, ymax=21, %
            xmin=0, xmax=1000, %
            xticklabels={},%
            ytick={0, 10, 20}, %
            name=first,%
            ]

            \coordinate (spypoint1) at (axis cs:30,10);
            \coordinate (magnifyglass1) at (axis cs:300,3.3);

            \coordinate (spypoint2) at (axis cs:850,3.3);
            \coordinate (magnifyglass2) at (axis cs:650,3.4);

        \end{axis}

        \begin{axis}[%
            height=4cm,%
            width=10cm, %
            ymin=-1, ymax=1, %
            xmin=0, xmax=1000, %
                at={(first.below south west)}, yshift=-0.1cm,  anchor=north west, %
            ]

            \coordinate (spypoint3) at (axis cs:30,0);
            \coordinate (magnifyglass3) at (axis cs:300,0.4);

            \coordinate (spypoint4) at (axis cs:850,-0.1);
            \coordinate (magnifyglass4) at (axis cs:600,-0.4);

        \end{axis}

        \spy [black, height=12mm, width=20mm, every spy on node/.append style={name=a1}] on (spypoint1)
            in node [fill=white,name=a] at (magnifyglass1);

        \spy [black, height=12mm, width=22mm] on (spypoint2)
            in node [fill=white,name=b] at (magnifyglass2);

        \spy [black, height=12mm, width=20mm, every spy on node/.append style={name=c1}] on (spypoint3)
            in node [fill=white,name=c] at (magnifyglass3);

        \spy [black, height=12mm, width=22mm] on (spypoint4)
            in node [fill=white,name=d] at (magnifyglass4);

        \end{scope}

        \draw[dashed] (a) -- (b);
        \draw[dashed] (c) -- (d);

        \draw[red] (a1) -- (c1);

    \end{tikzpicture}

\end{document} 

Answer 2

Basically this is the same answer as Christoph Frings' one, but I "optimized" it a bit. The main change is that I introduced the new style Name as a shorthand for every spy on node/.append style. For the rest have a look at the comments in the code.

---

\documentclass[border=2mm]{standalone}
\usepackage{pgfplots}
    \usetikzlibrary{spy}
    \tikzset{
        % define shorthand key/style to give spy-on node names
        Name/.style={
            every spy on node/.append style={
                name=#1,
            },
        },
    }
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            height=5cm,
            width=10cm,
            ymin=-1,ymax=21,
            xmin=0,xmax=1000,
            xticklabels={},
            ytick={0,10,20},
            name=first,
        ]
            \coordinate (spypoint1)     at (axis cs:30,10);
            \coordinate (magnifyglass1) at (axis cs:300,3.3);

            \coordinate (spypoint2)     at (axis cs:850,3.3);
            \coordinate (magnifyglass2) at (axis cs:650,3.4);
        \end{axis}

        \begin{axis}[
            height=4cm,
            width=10cm,
            ymin=-1,ymax=1,
            xmin=0,xmax=1000,
            at={(first.below south west)},
            yshift=-0.1cm,
            anchor=north west,
        ]
            \coordinate (spypoint3)     at (axis cs:30,0);
            \coordinate (magnifyglass3) at (axis cs:300,0.4);

            \coordinate (spypoint4)     at (axis cs:850,-0.1);
            \coordinate (magnifyglass4) at (axis cs:600,-0.4);
        \end{axis}

        % moved begin of scope here
        \begin{scope}[
            spy using outlines={%
                rectangle,
                magnification=4,
                connect spies,
                %
                % moved other common keys here
                black,
                height=12mm,
                width=20mm,
            },
        ]
            \spy [Name=a1] on (spypoint1)
                in node [fill=white,name=a] at (magnifyglass1);

            \spy [width=22mm] on (spypoint2)
                in node [fill=white,name=b] at (magnifyglass2);

            \spy [Name=c1] on (spypoint3)
                in node [fill=white,name=c] at (magnifyglass3);

            \spy [width=22mm] on (spypoint4)
                in node [fill=white,name=d] at (magnifyglass4);

        \end{scope}

        \draw[dashed] (a) -- (b);
        \draw[dashed] (c) -- (d);

        \draw[red] (a1) -- (c1);

    \end{tikzpicture}
\end{document}

Of course this also results in your desired output.