Drawing trees with xymatrix

Question

I would like to draw trees using xymatrix, however using for example

\xymatrix{\ar@{-}[dr] & & \ar@{-}[dl]\\ & \ar@{-}[d] & \\ & & }

returns the image

My problem with this is the fact that the edges don't join in the middle. Is there a way to correct to this (possibly not involving passing to tikz)?

Answer 1

There is a tree drawing package for linguistics based on xypic called xyling. It can do what you want, but since most linguistics trees assume node labels, and downward branches, drawing trees up will be a bit of hack. The main syntax uses xymatrix underlyingly, and implements some node commands and a \B command to make branches. The optional argument to the \B command is an adjustment to extend the node upwards to the empty node above.

\documentclass{article}

\usepackage{xyling}
\begin{document}
\Tree{ \B{dr} & & \B{dl} \\
        &\B[6]{d}\\
        &  &\\
}

\end{document}

Answer 2

Here's an example with forest. It is straightforward, once you have styles defined for the kinds of trees you need, to apply them. Trees can then be specified extremely concisely.

In this case, we define a style my tree and then apply it to a tree as follows:

\begin{forest}
  my tree
  [Aardvarks
    [Agglomeration
      [
        [
          [Aggrandisement]
          [Artificiality]
        ]
      ]
    ]
    [Anticipation
      [Ants]
      [Ant-Eaters, tikz={\draw [densely dashed, ->, red] (.parent anchor) [out=45, in=135] to (!s.parent anchor);}]
    ]
  ]
\end{forest}

To produce

The longest part of this is the arrow showing the ant-eaters eating the ants. But if you often need to illustrate that nature is red in tooth and claw, you could easily set up a style for this and just say e.g.

      [Ant-Eaters, tooth and claw]

to achieve the same result. But this is probably not a standard feature of linguistic trees and unnecessary here.

\documentclass[tikz,border=10pt,multi]{standalone}
\usepackage{forest}
\forestset{
  my tree/.style={
    for tree={
      parent anchor=north,
      child anchor=south,
      grow=90,
    },
    nice empty nodes up
  },
  nice empty nodes up/.style={% modified from page 52
    for tree={calign=fixed edge angles},
    delay={where content={}{shape=coordinate,for parent={for children={anchor=south}}}{}}
  }
}
\begin{document}
\begin{forest}
  my tree
  [Aardvarks
    [Agglomeration
      [
        [
          [Aggrandisement]
          [Artificiality]
        ]
      ]
    ]
    [Anticipation
      [Ants]
      [Ant-Eaters, tikz={\draw [densely dashed, ->, red] (.parent anchor) [out=45, in=135] to (!s.parent anchor);}]
    ]
  ]
\end{forest}
\end{document}