Two-loop electron self-energy with TikZ-Feynman

Question

I want to draw a simple self-energy two-loop graph such as

using TikZ-Feynman. I have a problem with the automatic alignment of the fermion lines. I have tried this:

\feynmandiagram[inline=(a), layered layout, horizontal=a to f] 
{
a--[plain, in=180, out=0, relative=true] b --[photon, half left] e --[plain, in=180, out=0, relative=true] f,
b--[plain, in=180, out=0, relative=true] c -- [photon, half left] d -- [plain, in=180, out=0, relative=true] e,
c -- [plain, in=180, out=0, relative=true] d,
}; 

Trying to force each vertex angle, but it gives me this:

Which is really bad. Is there some way to achieve this straight line without enforcing anything? Thank you!

Answer 1

I think that is one of the cases of the "When the Algorithm Isn’t Enough" category, but you can of course place the vertices manually.

\documentclass[tikz,border=3.14mm]{standalone}
\usepackage[compat=1.1.0]{tikz-feynman}
\begin{document}
\begin{tikzpicture}
\begin{feynman}
\vertex (a);
\vertex[right=1.5cm of a] (b);
\vertex[right=1.5cm of b] (c);
\vertex[right=1.5cm of c] (d);
\vertex[right=1.5cm of d] (e);
\vertex[right=1.5cm of e] (f);
\diagram*{(a)--[plain] (f),
(b) -- [photon, half left] (e), 
(c) -- [photon, half left] (d)
};
\end{feynman}
\end{tikzpicture}
\end{document}