QGDiagramStyle is an option for
QGCreateAmp, which specifies the QGRAF style file for
generating the diagrams. If you provide only the file name, they style
will be loaded from the standard directory containing model and style
files ($QGStylesDirectory).
If you specify the full path, the style file will be loaded from
there. The default value is a custom style file for FeynMP
"latex.sty". If the option value is set to an empty string,
no diagram file will be generated.