Position a tikz tree based on interior node

Question

How can I position a tikz tree based on an interior node, i.e. instead of defining the position of the root, defining the position of an interior node?

For example, consider the following code.

\documentclass[border=10pt,multi,tikz]{standalone}
\begin{document}
\begin{tikzpicture}
  \node {a}
    child {node at (1,0) {b}
      child {node {c}}
  };

  \node {x}
    child {node at (3,0) {y}
      child {node {z}}
  };
\end{tikzpicture}
\end{document}

This code does not produce the desired output, but it shows what I imagine.

Desired would be two trees where b and y are at (1,0) and (3,0), respectively, and the rest of the trees is normal around them, e.g., a is above b and c is below b respecting the level distance, etc.

Especially if a and c become larger trees, aligning the two trees at the inner node becomes tedious shifting work.

Answer 1

In order to prevent the nodes being placed as they are defined, you can use TikZ's graph drawing facilities with the trees layout library.

For example,

\IfFileExists{luatex85.sty}{\RequirePackage{luatex85}}{}
\documentclass[border=10pt,multi,tikz]{standalone}
\usetikzlibrary{graphs,graphdrawing}
\usegdlibrary{trees}
\begin{document}
\begin{tikzpicture}
  \graph [tree layout]
  {
    a -- { b[at={(1,0)}] , c }
  };
  \graph [tree layout]
  {
    x -- { y[at={(3,0)}] , z }
  };
\end{tikzpicture}
\end{document}

Compiling with LuaTeX gives the following result.

You can stick to the more verbose syntax if you prefer and still use the algorithmic graph-drawing facilities. This is just more concise.