\begin{fmfpicture}{-9,11}{2,12}{ttqq_gg-tth-{t}tt\q_schan} % padding (LTRB)
  \begin{fmfgraph*}(120,80) % dimensions (WH)
    % external vertices
    \fmfleft{i2,i1}
    \fmfright{o4,o3,o2,o1}
    \fmfshift{2 up}{o1,o2}
    \fmfshift{2 down}{o4,o3}
    % skeleton
    \fmf{gluon,t=1.9}{v1,i1}
    \fmf{gluon,t=1.9}{i2,v2}
    \fmf{fermion}{v1,o1}
    \fmf{fermion}{o4,v2}
    % ttbar -> h -> ttbar
    \fmf{fermion}{v2,v,v1}
    \fmf{dashes,t=0.9,label=h,l.s=left,l.d=4}{v,v'} % s-channel
    \fmf{fermion}{o3,v',o2}
    % labels
    \fmfv{l.a=158,l.d=6,l=$g$}{i1}
    \fmfv{l.a=-158,l.d=6,l=$g$}{i2}
    \fmfv{l.a=22,l.d=6,l=t}{o1}
    \fmfv{l.a=25,l.d=4,l=t}{o2}
    \fmfv{l.a=-20,l.d=4,l=$\overline{\mathrm{t}}$}{o3}
    \fmfv{l.a=-22,l.d=6,l=$\overline{\mathrm{t}}$}{o4}
  \end{fmfgraph*}
\end{fmfpicture}
\end{document}