qtree - labeling levels of a tree

Question

In either tikz-qtree or qtree, is it possible to label or number different levels of a (vertical) tree? Such labels could be at the right or the left side of the whole tree.

Answer 1

You can use \nodes to name some special nodes and allow getting access to their position in the tree and then use the named nodes to place the text. A little example placing text at both sides:

\documentclass[border=3pt]{standalone}
\usepackage{tikz-qtree}

\begin{document}

\begin{tikzpicture}
\Tree 
[.\node (level0-right) {S}; 
  [.NP 
    [.Det \node (level3-left) {the}; 
    ] 
    [.N cat 
    ] 
  ]
  [.\node (level1-right) {VP}; 
    [.V sat 
    ]
    [.\node (level2-right) {PP}; 
      [.P on 
      ]
      [.\node (level3-right) {NP}; 
        [.Det the 
        ] 
        [.\node (level4-right) {N}; \node (level5-right) {mat}; 
        ] 
      ]
    ] 
  ] 
]
\foreach \Value/\Text in {0/{Texto 1},1/{Texto 2},2/{Texto 3},3/{Texto 4},4/{Texto 5},5/{Texto 6}}
{  
  \node[anchor=west] 
    at ([xshift=1cm]{level5-right}|-{level\Value-right}) 
    {\Text};
  \node[anchor=east] 
    at ([xshift=-1cm]{level3-left}|-{level\Value-right}) 
    {\Text};
}
\end{tikzpicture}

\end{document}

Answer 2

Just for the heck of it, here's Gonzalo Medina's example tree with prooftrees:

\documentclass[tikz,border=10pt]{standalone}
\usepackage{prooftrees}% version 0.8
\begin{document}
\begin{prooftree}
  {
    single branches
  }
  [S, just=explan 1
    [NP, just=explan 2
      [Det [the]]
      [N [cat]]
    ]
    [VP
      [V [sat]]
      [PP, just=explan 3
        [P [on]]
        [NP, just=explan 4
          [Det, just=explan 5 [the, just=explan 6]]
          [N [mat]]
        ]
      ]
    ]
  ]
\end{prooftree}
\end{document}

Note that the numbering on the left is automatic. (It can be switched off, but it is default.) Bear in mind, though, that the package is really designed specifically for logical proofs and not for other kinds of trees.

EDIT

Here's the same tree with some overlay specifications in Beamer:

This is using the standard definitions I use for tikzpictures generally and forest trees, in particular:

\documentclass{beamer}
\usepackage{prooftrees}% version 0.8
\tikzset{% set up for transitions using tikz with beamer overlays
    invisible/.style={opacity=0,text opacity=0},
    visible on/.style={alt=#1{}{invisible}},
    alt/.code args={<#1>#2#3}{%
      \alt<#1>{\pgfkeysalso{#2}}{\pgfkeysalso{#3}} % \pgfkeysalso doesn't change the path
    },
    transparent/.style={opacity=0.1,text opacity=0.1},
    opaque on/.style={alt=#1{}{transparent}},
    alerted/.style={color=red},
    alert on/.style={alt=#1{alerted}{}},
}
\forestset{%
  visible on/.style={%
        for tree={%
          /tikz/visible on={#1},
          edge={/tikz/visible on={#1}}}},
  opaque on/.style={%
        for tree={%
          /tikz/opaque on={#1},
          edge={/tikz/opaque on={#1}}}},
  alerted on/.style={%
        for tree={%
          /tikz/alerted on={#1},
          edge={/tikz/alerted on={#1}}}},
}%
\begin{document}
\begin{frame}{A Tree}
\begin{prooftree}
  {
    single branches,
  }
  [S, just=explan 1
    [NP, just=explan 2, alert on=<4->, for children={opaque on=<5->}
      [Det [the]]
      [N [cat]]
    ]
    [VP
      [V [sat]]
      [PP, just=explan 3
        [P [on]]
        [NP, just=explan 4, alert on=<2>
          [Det, just=explan 5, opaque on=<3-> [the, just=explan 6]]
          [N [mat]]
        ]
      ]
    ]
  ]
\end{prooftree}
\end{frame}
\end{document}