alignment issue with tikz graph

Question

Consider

\documentclass[11pt]{article}
\usepackage{tikz-feynman}
\begin{document}
\begin{tikzpicture}
\graph[layered layout, horizontal=a to b] {
  q -- a,
  qb -- a,
  a -- b -- c -- g1,
  c -- g2,
  b -- g3,
};
\end{tikzpicture}
\end{document}

With LuaTeX, Version 0.95.0 (TeX Live 2016) on a mac running macOS Sierra, I get this

But I would like g1, g2 and g3 to be vertically aligned. How would I do that? I tried to use {[same layer] g1, g2, g3} but then it gives

I would like to keep the angles exactly like those of the first diagram. With just the b--g3 line longer to put g3 aligned with g1 and g2. I even tried g3 [below=of g2] but it does not work.

Answer 1

You can force a node to be placed at a specific position on the canvas, after the node placement calculations have been completed. This is done with the regardless at syntax, which are described on p380 of the TikZ manual (v3.0.1a).

Node q is placed at the origin (0,0). With the graph set on a 1cm grid, it is then a simple matter of determining the appropriate co-ordinate for node g3. We find that the co-ordinate is (4,-2), which aligns g3 with g1 and g2.

So we can then write: b -- g3[regardless at={(4,-2)}] to add the node at the desired position. Following is the result, which incorporates a background grid (How can I show coordinates by grid in TikZ automatically?) to illustrate the positioning of node g3 at (4,-2).

This is a MWE, modified from the original to show the libraries that are specifically required. The Tikz-Feynman package is not loaded since none of its macros are used. Note that use of the TikZ graph drawing libraries, requires lualatex to compile the code.

% !TeX TS-program = lualatex

\documentclass[tikz,border=5pt]{standalone}
\usepackage{luatex85}

\usetikzlibrary{
    graphs,
    graphdrawing,
}
\usegdlibrary{
    layered,   % requied for layered layout
}

% https://tex.stackexchange.com/a/39698
\makeatletter
\def\grd@save@target#1{%
    \def\grd@target{#1}}
\def\grd@save@start#1{%
    \def\grd@start{#1}}
\tikzset{
    grid with coordinates/.style={
        to path={%
            \pgfextra{%
                \edef\grd@@target{(\tikztotarget)}%
                \tikz@scan@one@point\grd@save@target\grd@@target\relax
                \edef\grd@@start{(\tikztostart)}%
                \tikz@scan@one@point\grd@save@start\grd@@start\relax
                \draw[minor help lines] (\tikztostart) grid (\tikztotarget);
                \draw[major help lines] (\tikztostart) grid (\tikztotarget);
                \grd@start
                \pgfmathsetmacro{\grd@xa}{\the\pgf@x/1cm}
                \pgfmathsetmacro{\grd@ya}{\the\pgf@y/1cm}
                \grd@target
                \pgfmathsetmacro{\grd@xb}{\the\pgf@x/1cm}
                \pgfmathsetmacro{\grd@yb}{\the\pgf@y/1cm}
                \pgfmathsetmacro{\grd@xc}{\grd@xa + \pgfkeysvalueof{/tikz/grid with coordinates/major step}}
                \pgfmathsetmacro{\grd@yc}{\grd@ya + \pgfkeysvalueof{/tikz/grid with coordinates/major step}}
                \foreach \x in {\grd@xa,\grd@xc,...,\grd@xb}
                \node[anchor=north,color=lightgray] at ([yshift=-0.15cm]\x,\grd@ya) {\pgfmathprintnumber{\x}};
                \foreach \y in {\grd@ya,\grd@yc,...,\grd@yb}
                \node[anchor=east,color=lightgray] at ([xshift=-0.15cm]\grd@xa,\y) {\pgfmathprintnumber{\y}};
            }
        }
    },
    minor help lines/.style={
        help lines,
        step=\pgfkeysvalueof{/tikz/grid with coordinates/minor step},
        minor line width/.initial=0.25pt,
    },
    major help lines/.style={
        help lines,
        line width=\pgfkeysvalueof{/tikz/grid with coordinates/major line width},
        step=\pgfkeysvalueof{/tikz/grid with coordinates/major step},
        line cap=round,
        color=lightgray,
    },
    grid with coordinates/.cd,
    minor step/.initial=.2,
    major step/.initial=1,
    major line width/.initial=0.5pt,
}
\makeatother
\begin{document}

\begin{tikzpicture}[
font=\tiny,
color=blue
]

\draw (0,-2) to[grid with coordinates] (4,1);

\graph[
layered layout, 
horizontal=a to b,
] {
  {q,qb} -- a,
  a -- b -- c,
  c -- {g1,g2},
  b -- g3[regardless at={(4,-2)}],
};
\end{tikzpicture}
\end{document}