如何在序列图下方添加标签？

Question

我还没有找到在序列图下添加标签（如图片中）的方法。可以吗？我想在文本中引用图表，但我不知道如何为每个序列图显示标签。TIA

例如

\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{pgf-umlsd}

\newcommand{\newthreadShift}[4][gray!30]{
  \newinst[#4]{#2}{#3}
  \stepcounter{threadnum}
  \node[below of=inst\theinstnum,node distance=0.8cm] (thread\thethreadnum) {};
  \tikzstyle{threadcolor\thethreadnum}=[fill=#1]
  \tikzstyle{instcolor#2}=[fill=#1]
}

\begin{sequencediagram}
    \centering
    \newthread{A}{Alice}
    \newthreadShift{B}{Bob}{3cm}
    
    \begin{messcall}{A}{$C, Id$}{B}{}
    \end{messcall}
    
        \begin{messcall}{B}{$C, Id$}{A}{}
   \end{messcall}
    
\end{sequencediagram}

如下图所示：

我如何在第 V 部分之前的图表（即图 1）中添加标签。

Answer 1

为了好玩，但你可能喜欢：

\documentclass[margin=3mm, many]{standalone}
\usepackage[T1]{fontenc}

\usepackage{tikz}
\usetikzlibrary{arrows.meta,
                positioning,
                quotes
            }
\usepackage{tikzpeople}

\begin{document}

    \begin{tikzpicture}[
node distance = 55mm,
            > = Straight Barb,
    ms/.style = {minimum size=7mm},
    ts/.style = {transform canvas={yshift=#1}},
     N/.style = {draw, fill=gray, inner sep=0pt,
                 minimum height=22mm, minimum width=2mm,
                 node contents={},
                },
every edge quotes/.style = {auto, font=\small, 
                            text=black, inner sep=1pt}
                        ]
\node (s)   [alice,ms]              {Alice};
\node (r)   [bob,  ms,right=of s]   {Bob};
%
\node (L) [N, below right=2mm of s.north east] (L);
\node (R) [N, below  left=2mm of r.north west] (R);
    \draw[dotted]   (L.south) edge ++ (0,-0.5)
                    (R.south) edge ++ (0,-0.5);

\draw [->,ts= 5ex] (L) edge["{$C,Id$}"] (R);
\draw [<-,ts= 2ex] (L) edge["{$C,Id$}"] (R);
\draw [->,ultra thick, gray,
        ts=-2ex] (L) edge["data?"]    (R);
    \end{tikzpicture}
\end{document}