Dividing Tikz-node graph into rows

Question

I have a script that takes some input data and outputs tikz-code that creates a visual representation of this data in the form of a node graph. The ouput of the current script using example data looks as follow at the moment:

\documentclass{article}

\usepackage{tikz}

\begin{document}

    \begin{tikzpicture}[every node/.style={{inner sep=0.7em}}]
        \node at (6em, 0em) (L1N1) [anchor=north, text width=4em]{\begin{center}N1\end{center}};
        \node at (0em, -9em) (L2N1) [anchor=north, text width=4em]{\begin{center}N1\end{center}};
        \node at (6em, -9em) (L2N2) [anchor=north, text width=4em]{\begin{center}N2\end{center}};
        \node at (12em, -9em) (L2N3) [anchor=north, text width=4em]{\begin{center}N3\\L1\\L2\\L3\\L4\\L5\\L6 \end{center}};
        \node at (0em, -18em) (L3N1) [anchor=north, text width=4em]{\begin{center}N1\end{center}};
        \node at (12em, -18em) (L3N2) [anchor=north, text width=4em]{\begin{center}N2\end{center}};
        \node at (6em, -27em) (L4N1) [anchor=north, text width=4em]{\begin{center}N1\end{center}};
        \path [-] (L1N1) edge (6em, -7.0em);
        \path [-] (6em, -7.0em) edge (6em, -7.0em);
        \path [->] (6em, -7.0em) edge (L2N2);
        \path [-] (L2N1) edge (0em, -16.0em);
        \path [-] (0em, -16.0em) edge (0em, -16.0em);
        \path [-] (L2N2) edge (6em, -16.0em);
        \path [-] (6em, -16.0em) edge (12em, -16.0em);
        \path [-] (L2N3) edge (12em, -16.0em);
        \path [-] (12em, -16.0em) edge (12em, -16.0em);
        \path [->] (0em, -16.0em) edge (L3N1);
        \path [->] (12em, -16.0em) edge (L3N2);
        \path [->] (12em, -16.0em) edge (L3N2);
        \path [-] (L3N1) edge (0em, -25.0em);
        \path [-] (0em, -25.0em) edge (6em, -25.0em);
        \path [-] (L3N2) edge (12em, -25.0em);
        \path [-] (12em, -25.0em) edge (6em, -25.0em);
        \path [->] (6em, -25.0em) edge (L4N1);
        \path [->] (6em, -25.0em) edge (L4N1);
    \end{tikzpicture}

\end{document}

The generated graph looks like this:

The image illustrates the problem I have. As can be seen from the graph, I visualize it as several rows of Nodes with connections between nodes in different rows. Currently my node positions are computed entirely in the script but I do think that I need to go away from this, because I have a problem with not knowing the size of the nodes and the rightmost node in the second row is far too tall for the coordinates given, making the result a mess.

Is it possible to compute the Y-coordinate of each row automatically in tikz (as an offset from the bottom of the previous row)? I do believe I can use the "calc" tikzlibrary to get the bottom-most Y-coordinate of a given node but I have not been able to find a way to get the bottom-most Y-coordinate of several Nodes. Is there a way to do that or is there another approach I should take to solve this problem?

Here is a rough sketch on how I would like the end result to look, with the third line being offset based on the lower most point of the nodes of the second line:

I cannot generate the final representation of the graph externally, as I will need to put page-references both from and to the contents of some of the nodes.

Thank you for any help!

Answer 1

Edit: Some remarks:

• in my proposition I follow to your MWE, which is quite close to desired result
• main differences between them are use of the relative positioning (by employ positioning library) of nodes and added two auxiliary coordinates which are served for node connections and positions
• for nodes are used common style defined as every node
• number commands for arrows are reduced to only to shown on images (five)

\documentclass[tikz, margin=3mm]{standalone}
\usetikzlibrary{positioning}

\begin{document}
    \begin{tikzpicture}[
    node distance = 4mm and 12mm,
every node/.style = {minimum size=1.2em, align=center, outer sep=0pt},
                        ]
\node (L1N1)    {N1};
\node (L2N1)    [below  left=of L1N1]   {N1};
\node (L2N2)    [below=of L1N1]         {N2};
\node (L2N3)    [below right=of L1N1]   {N3\\L1\\L2\\L3\\L4\\L5\\L6 };
%
\coordinate[below=of L2N3] (aux1);
%
\node (L3N1)    [below=of L2N1 |- aux1] {N1};
\node (L3N2)    [below=of aux1]         {N2};
%
\coordinate[below=of L1N1 |- L3N1.south]    (aux2);
%
\node (L4N1)    [below=of aux2]   {N1};
%
\draw[->]   (L1N1) -- (L2N2);
\draw[->]   (L2N1) -- (L3N1);
\draw[->]   (L2N3) -- (L3N2);
%
\draw[->]   (aux2) -- (L4N1);
\draw       (L2N2) |- (aux1);
%
\draw       (L3N1) |- (aux2) -| (L3N2);
    \end{tikzpicture}
\end{document}