# Alignment of equations that contain Feynman diagrams

I want to insert a Feynman diagram into an equation, but when I do so, I find that the Feynman diagram and the rest of the equation are awkwardly aligned. The picture should make it pretty clear what I mean. Is there a way to center the expression in this equation with the Feynman diagram?

\documentclass{article}

\usepackage{amsmath}
\usepackage{feynmf}

\begin{document}

\begin{fmffile}{test}
$$\begin{fmfgraph*}(65,50) \fmfleft{i1,i2} \fmfright{o1,o2} \fmf{plain}{i1,v,o1} \fmf{plain}{i2,v,o2} \end{fmfgraph*}=-i\lambda$$
\end{fmffile}

\end{document}


Use gathered:

\documentclass{article}

\usepackage{amsmath}
\usepackage{feynmp-auto} % or feynmf

\begin{document}

\begin{fmffile}{test}
$$\begin{gathered} \begin{fmfgraph*}(65,50) \fmfleft{i1,i2} \fmfright{o1,o2} \fmf{plain}{i1,v,o1} \fmf{plain}{i2,v,o2} \end{fmfgraph*} \end{gathered}=-i\lambda$$
\end{fmffile}

\end{document}