Putting a title on a Feynman diagram with feynmp

Question

I would like to put an overall title for a FeynMP diagram, as it is going to be put in a figure side by side with many other Feynman diagrams. I would like it to look something like this:

The option to put it either top or bottom would be also welcome. The source without the title is as follows:

\documentclass{article}
\usepackage{feynmp}
\DeclareGraphicsRule{*}{mps}{*}{}

\begin{document}
\thispagestyle{empty}
\unitlength = 1mm
%lb - left bottom
%rt - right top
\begin{fmffile}{gluon}
\begin{fmfgraph*}(30,30)
  \fmfleft{lb,lt}
  \fmflabel{Left bottom}{lb}
  \fmflabel{Right bottom}{rb}
  \fmfright{rb,rt}
  \fmflabel{Right top}{rt}
  \fmflabel{Left top}{lt}
  \fmf{fermion}{lb,v1}
  \fmf{fermion}{v1,rt}
  \fmf{dashes_arrow}{rb,v1}
  \fmf{fermion}{v1,lt}
  \fmfdot{v1}
\end{fmfgraph*}
\end{fmffile}
\write18{mpost gluon}
\end{document}

Any information will be appreciated!

Answer 1

I've come up with two ways to solve the problem:

1) Add the title to the tex file which generates the diagram. The advantage is that the title is the same size as the labels on the diagram. Code:

\documentclass{article}
\usepackage{feynmp}
\DeclareGraphicsRule{*}{mps}{*}{}

\begin{document}
\thispagestyle{empty}
\unitlength = 1mm
%lb - left bottom
%rt - right top
%
\begin{center}
TITLE
\vspace{3ex} %space that is 3 'x' letters high
\end{center}
%
\begin{fmffile}{gluon}
\begin{fmfgraph*}(30,30)
  \fmfleft{lb,lt}
  \fmflabel{Left bottom}{lb}
  \fmflabel{Right bottom}{rb}
  \fmfright{rb,rt}
  \fmflabel{Right top}{rt}
  \fmflabel{Left top}{lt}
  \fmf{fermion}{lb,v1}
  \fmf{fermion}{v1,rt}
  \fmf{dashes_arrow}{rb,v1}
  \fmf{fermion}{v1,lt}
  \fmfdot{v1}
\end{fmfgraph*}
\end{fmffile}
\write18{mpost gluon}
\end{document}

2) After generating the PDFs, put them into subfigures, as described in the wikibooks latex tutorial.

Thanks again to egreg and Steven B. Segletes!