Tikz trees: How to add label to dashed information set

Question

I searched online and in stack overflow for how to add a label to a straight dashed information set but was not successful. I see suggestions to add a label to a curved dashed information set but when I add them to my code for a straight information set, they do not work.

Does anyone have any suggestions? I attached an image of what I am trying to do. Thank you in advance.

I tried this but it did not work: \draw[dashed, label=above right:{my label}](0-1)to(0-2);

This did not work too: \node at($(0-1)!.5!(0-2)$){my label};

Here is my code.

\begin{tikzpicture} [scale=1.5] %[scale=1.5,font=\footnotesize]
    \tikzstyle{solid node} =[circle,draw,inner sep=1.5,fill=black]
    \tikzstyle{hollow node}=[circle,draw,inner sep=1.5]
    \tikzstyle{level 1}=[level distance=15mm,sibling distance=3.5cm]
    \tikzstyle{level 2}=[level distance=15mm,sibling distance=2.5cm]
    \tikzstyle{level 3}=[level distance=15mm,sibling distance=1cm]
\node(0)[hollow node,label=above:{  }]{} 
child{node[solid node]{}
    child{node[hollow node,label=below:{$(1,2)$}]{} edge from parent node[left]{a}}
    child{node[hollow node,label=below:{$(1,-1)$}]{} edge from parent node[left]{b}}
    %child{node[hollow node,label=below:{$(0,2)$}]{} edge from parent node[right]{$E$}}
    edge from parent node[left,xshift=-5]{action1}
}
child{node[solid node]{}
    child{node[hollow node,label=below:{$(2,2)$}]{} edge from parent node[left]{a}}
    child{node[hollow node,label=below:{$(1,3)$}]{} edge from parent node[right]{b}}
    edge from parent node[right,xshift=5]{action2}
};
\draw[dashed, label=above right:{my label}](0-1)to(0-2);
\end{tikzpicture}

Answer 1

Not the best answer, but it solves your problem.

Creates a node on the dashed line and puts the text on the node. Whit yshift and xshiftto control the text position.

\begin{tikzpicture} [scale=1.5] %[scale=1.5,font=\footnotesize]
   \tikzstyle{solid node} =[circle,draw,inner sep=1.5,fill=black]
   \tikzstyle{hollow node}=[circle,draw,inner sep=1.5]
   \tikzstyle{level 1}=[level distance=15mm,sibling distance=3.5cm]
   \tikzstyle{level 2}=[level distance=15mm,sibling distance=2.5cm]
   \tikzstyle{level 3}=[level distance=15mm,sibling distance=1cm]
\node(0)[hollow node,label=above:{  }]{} 
child{node[solid node]{}
    child{node[hollow node,label=below:{$(1,2)$}]{} edge from parent node[left]{a}}
    child{node[hollow node,label=below:{$(1,-1)$}]{} edge from parent node[left]{b}}
    %child{node[hollow node,label=below:{$(0,2)$}]{} edge from parent node[right]{$E$}}
    edge from parent node[left,xshift=0.35cm,yshift = 0.55cm,rotate  = 38]{action1}
}
   child{node[solid node]{}
    child{node[hollow node,label=below:{$(2,2)$}]{} edge from parent node[left]{a}}
    child{node[hollow node,label=below:{$(1,3)$}]{} edge from parent node[right]{b}}
    edge from parent node[right,xshift=5]{action2}
};
   \draw[dashed] (0-1) node[midway, yshift = -1.3cm] {$label$} to(0-2)  ;
\end{tikzpicture}

The output is this:

If you want to rotate the text in lines use the comand rotate, an example in "action1".

Answer 2

Here is a solution with forest.

\documentclass{article}
\usepackage{forest}
\tikzset{open/.style={draw, circle, minimum width=1.5mm, inner sep=0},
    closed/.style={open, fill}}
\begin{document}
\begin{forest}
for tree={if level=1{closed}{open},
    s sep=2cm, l sep=1.5cm
}
[
  [, edge label={node[midway, left=4pt]{action1}}, name=A1
    [, label={south:$(1,2)$}, edge label={node[midway, left]{a}}]
    [, label={south:$(1,-1)$}, edge label={node[midway, left]{b}}]
  ]
  [, edge label={node[midway, right=4pt]{action2}}, name=A2
    [, label={south:$(2,2)$}, edge label={node[midway, left]{a}}]
    [, label={south:$(1,3)$}, edge label={node[midway, right]{b}}]
  ]
]
\drawdashed--node[above]{label}(A2);
\end{forest}
\end{document}

Answer 3

• Label to straight line is simple to add, For this you can chose between two options:
  • by use of node: \draw (<coordonate 1>) -- node[above] {text} (<coordinate 2>);
  • by use of the quotes library: \draw (<coordonate 1>) to["text"](<coordinate 2>);.
• in MWE (Minimal Working Example) below is considered second possibility.
• Also with `forest˛but with more sophisticated forest diagram preamble and simpler diagram body code:

\documentclass[border=3.141592]{standalone}
\usepackage{forest}
\usetikzlibrary{quotes}
\begin{document}
    \begin{forest}
for tree={
% nodes
    circle, draw,
    minimum size=2pt, inner sep=0pt,
    font=\small,
% tree
    l sep = 12mm,
    s sep = 11mm,
 if level = 1{fill}{},
        },
%% edge labels
/tikz/ELS/.style = {% Edge Label Style
                    pos=0.5,
                    node font=\scriptsize, anchor=#1},
       EL/.style = {%Edge Label
                    if n=1{edge label={node[ELS=east]{#1}}}
                          {edge label={node[ELS=west]{#1}}}
                    },
      lbl/.style = {label={south:$#1$}}
%%%% diagram body
[
    [, EL=action 1, name=a
        [, lbl={(1, 2)}, EL=a]
        [, lbl={(1,-1)}, EL=b]
    ]
    [, EL=action 2, name=b
        [, lbl={(2,2)}, EL=a]
        [, lbl={(1,3)}, EL=b]
    ]
]
\draw[densely dashed, very thin,font=\scriptsize]
    (a) to["label"] (b);  % <---
    \end{forest}
\end{document}