\begin{xy}
% \xyWARMprocessMo{\Fname}{}
 \xyShowAllMarkedPoints{}{\Fname}{\grExt}%
 \xyMarkedImport{\Fname}% repeat name else pdfTeX crashes
 \xyMarkedPos{x}*++!D{x}
 \xyMarkedPos{phi}*++!D!L(.5){\varphi_t(x)}
 \xyMarkedPos{M}*+!LD{M}
 \xyMarkedPos{u}*+!UL{u(\varphi_t(x),t)}
 \xyMarkedPos{traj}*++!L!U(.5){\parbox{10em}{trajectory of\\fluid particle\\
 moving through\\$x$ at time $\mathrm{t}=t$}}
\end{xy}