[Xy-pic] inexact end points of arrows in \xymatrix (data flow diagram)

[Sven Hartrumpf]

(I posted the following in comp.text.tex but received no solution.
So I am now sending this to the specialists :-) - Hello.)

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I like the Xy-pic package (version 3.7 <1999/02/16>) very much,
especially the \xymatrix command.
But now I am faced with the problem that arrows are inexact, i.e.
they don't always start or end where they should (at the boundary of
an object).
Here is a minimal example, a data flow diagram in Xy-pic:

\documentclass{article}

\usepackage[arrow,frame,matrix]{xy}

\newcommand{\datacoll}[1]{*[F_)][F^)]{\parbox{8 em}{\begin{center}#1\end{center}}}}
\newcommand{\funcoll}[1]{*[Fo]{\parbox{8 em}{\begin{center}#1\end{center}}}}
\newcommand{\extent}[1]{*[F-]{\parbox{8 em}{\begin{center}#1\end{center}}}}
\newcommand{\flow}[1]{\ar[#1]}
\newcommand{\bflow}[1]{\ar@{<->}[#1]}

\begin{document}
\mbox{\xymatrix{
 &
   \datacoll{corpus}
   \flow{r}
   \flow{d}
 &
   \extent{annotators}
   \flow{r}
 &
   \datacoll{annotations (keys)}
   \flow{d}
\\
 &
   \funcoll{train and test set sampler (1)}
   \flow{d}
   \flow{dr}
 &
 &
   \funcoll{train and test annotation sampler (1)}
   \flow{d}
   \flow{dr}
\\
 &
   \datacoll{train set}
 &
   \datacoll{test set}
 &
   \datacoll{train annotations (1)}
 &
   \datacoll{test annotations}
\\
}}

\end{document}

The output from pdflatex can be seen at:
http://pi7.fernuni-hagen.de/hartrumpf/bugxy.pdf (bugxy.tex)
The problem also exists in the DVI file.

All hints welcome.


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(Adding dvips option for .ps file does not help,
see http://pi7.fernuni-hagen.de/hartrumpf/bugxy.ps)