\begin{xy}
% \xyWARMprocessMo{\Fname}{}
 \xyShowAllMarkedPoints{}{\Fname}{\grExt}%
 \xyMarkedImport{\Fname}% repeat name else pdfTeX crashes
 \xyMarkedPos{Sigma}*+!LU{\Sigma}
 \xyMarkedPos{C}*++++!L{C}
 \xyMarkedPos{n}*++++!L{\vec{n}}
\end{xy}