请协助将标签放置在树线旁边，并增加节点 J 的大小

Question

\documentclass[12pt]{article}
\usepackage{syntonly}
\usepackage{geometry}

\usepackage{caption}
\usepackage{subcaption}

\usepackage{tikz}
\usetikzlibrary{arrows.meta,
                positioning}
\tikzset{node distance = 13mm and 11mm,
           C/.style = {circle,draw, fill=blue!20,
                       minimum size=1cm, inner sep=0pt},
         dot/.style = {circle, fill=black, inner sep=2pt,
                       node contents={} },
        }

\begin{document}
    \begin{figure}[ht]
    
\begin{subfigure}[b]{0.3\textwidth}
\caption{}
\label{subfig:1}
    \begin{tikzpicture}
\node (j) [C, label=$j$] {};
\node [dot];
\node (i) [C, below right = of j, label=below:$i$] {};
\node (k) [C, above right = of i, label=$k$] {};
%
\draw[thick]    (i) -- node {$y_i$}(i |- j)   % <---
                (j.center) -- node {$x_i$}(k);
    \end{tikzpicture}
\end{subfigure}
\hfill
\begin{subfigure}[b]{0.3\textwidth}
\caption{}
\label{subfig:2}
    \begin{tikzpicture}
\node (j) [C, label=$j$] {};
\node [dot];
\node (k) [C, below right = of j, label=below:$k$] {};
\node (i) [C, above right = of k, label=$i$] {};
%
\draw[thick]    (k) -- node {$y_k$}(k |- j)   % <---
                (j.center) -- node {$x_k$}(i);
    \end{tikzpicture}
\end{subfigure}
\hfill
\begin{subfigure}[b]{0.3\textwidth}
\caption{}
\label{subfig:3}
    \begin{tikzpicture}
\node (i) [C, label=$i$] {};
\node [dot];
\node (j) [C, below right = of i, label=below:$j$] {};
\node (k) [C, above right = of j, label=$k$] {};
%
\draw[thick]    (j) -- node {$y_j$}(j |- i)   % <---
                (i.center) --node {$x_j$} (k);
    \end{tikzpicture}
\end{subfigure}

\caption{Jacobi coordinates for a three-body system}
\label{fig:jacobi}
    \end{figure}
\end{document}

Answer 1

这就是你想要的吗？

\documentclass[12pt]{article}
\usepackage{syntonly}
\usepackage{geometry}

\usepackage{caption}
\usepackage{subcaption}

\usepackage{tikz}
\usetikzlibrary{arrows.meta,
    positioning}
\tikzset{node distance = 13mm and 11mm,
    C/.style = {circle,draw, fill=blue!20,
        minimum size=1cm, inner sep=0pt},
    Cj/.style = {circle,draw, fill=blue!20,
        minimum size=1.4cm, inner sep=0pt},
    dot/.style = {circle, fill=black, inner sep=2pt,
        node contents={} },
}

\begin{document}
    \begin{figure}[ht]
        
        \begin{subfigure}[t]{0.3\textwidth}
            \caption{}
            \label{subfig:1}
            \begin{tikzpicture}
                \node (j) [Cj, label=$j$] {};
                \node [dot];
                \node (i) [C, below right = of j, label=below:$i$] {};
                \node (k) [C, above right = of i, label=$k$,yshift=5pt] {};
                %
                \draw[thick]    (i) -- node[left] {$y_i$}(i |- j)   % <---
                (j.center) -- node[midway,above,xshift=7pt] {$x_i$}(k);
            \end{tikzpicture}
        \end{subfigure}
        \hfill
        \begin{subfigure}[t]{0.3\textwidth}
            \caption{}
            \label{subfig:2}
            \begin{tikzpicture}
                \node (j) [Cj, label=$j$] {};
                \node [dot];
                \node (k) [C, below right = of j, label=below:$k$] {};
                \node (i) [C, above right = of k, label=$i$,yshift=5pt] {};
                %
                \draw[thick]    (k) -- node[left] {$y_k$}(k |- j)   % <---
                (j.center) -- node[midway,above,xshift=7pt] {$x_k$}(i);
            \end{tikzpicture}
        \end{subfigure}
        \hfill
        \begin{subfigure}[t]{0.3\textwidth}
            \caption{}
            \label{subfig:3}
            \vspace{7pt}
            \begin{tikzpicture}
                \node (i) [C, label=$i$] {};
                \node [dot];
                \node (j) [Cj, below right  = of i,label=below:$j$] {};
                \node (k) [C, above right = of j, label=$k$] {};
                %
                \draw[thick]    (j) -- node[left] {$y_j$}(j |- i)   % <---
                (i.center) --node[midway,above,xshift=7pt] {$x_j$} (k);
            \end{tikzpicture}
        \end{subfigure}
        
        \caption{Jacobi coordinates for a three-body system}
        \label{fig:jacobi}
    \end{figure}
\end{document}

输出：