Multi-rooted Tree-like Structures and Nodes with Multiple Parents in LaTeX

Question

The qtree package seems to facilitate the drawing of simple trees only. Is there a way to incorporate:

1. Multiple roots &
2. Nodes with multiple parents

in a tree in LaTeX?

Answer 1

Multi-dominance "trees" are not really trees, but graphs, and the standard tree drawing packages are really not designed to handle them. Depending on the complexity of the tree, it's possible to draw these trees, however, but it requires some manual intervention. I would use one of the TikZ based tree drawing packages for this rather than qtree. The two best such packages are tikz-qtree and very powerful forest package. The general method for achieving multi-dominance is the same: you draw an empty edge and node, and then manually draw the extra branch to the relevant node that has multiple parents.

Using tikz-qtree

\documentclass{article}
\usepackage{tikz-qtree,tikz-qtree-compat}


\begin{document}
% Some multi-dominance structures using qtree and forest
% We fake the multi-dominance with an empty edge and node and then
% a regular tikz \draw between the node and its extra parent
\begin{tikzpicture}[sibling distance=.25cm]
\Tree [.Y
        [.Z ]
        [.\node (X) {X}; 
           [.A [.B ] [.\node (C) {C}; ] ]
           [.\node (A2) {A};
             \edge[draw=none]; {} \edge; {B} ]]]] 
\draw (A2.south) -- (C.north);

\end{tikzpicture}
\end{document}

Using forest

\documentclass{article}
\usepackage{forest}
\forestset{qtree edges/.style={for tree=
    {parent anchor=south, child anchor=north}}}

\begin{document}
\begin{forest}
qtree edges
[Y
[Z ]
[X,name=X [A  [B ] [C, name=C ]]
   [A, name=A2  [, no edge ] [B ]]]]
\draw (A2.south) -- (C.north);
\end{forest}
\end{document}

Graphs with tkz-graph

For more complicated graphs, it's likely better to use a package designed specifically for graphs. Alain Matthes' tkz-graph package does just that. Here's a more complex example using that package. The documentation is in French, but there are plenty of examples to work from, and all the command syntax is English-based. Note also that tikz-qtree and tkz-graph cannot be used together because of command name clashes, but there is a patch to be found here:

How to use tkz-graph and tikz-qtree without conflict?.

\documentclass{article}
% a more complicated graph using tkz-graph
% depending on the actual graph this could be partially automated
% with tikz \for loops
\usepackage{tkz-graph}
\renewcommand*{\EdgeLineWidth}{0.15pt}

\begin{document}
\begin{tikzpicture}
\GraphInit[vstyle=Empty]
\Vertex{Y}
\Vertex[x=2,y=0]{Q}
\Vertex[x=1,y=-2]{X}
\Vertex[x=-1,y=-2]{Z}
\Vertex[x=3,y=-2]{R}
\Vertex[x=0,y=-3]{A}
\Vertex[x=2,y=-3]{B}
\Vertex[x=-1,y=-4]{C}
\Vertex[x=1,y=-4]{D}
\Vertex[x=3,y=-4]{E}
\Edges(Z,Y,X,Q,R) 
\Edges(A,X,B)
\Edges(C,A,D,B,E)

\end{tikzpicture}
\end{document}