[Xy-pic] shorter arrows

[Jonas Frey]

Hi,

I'm looking for a way to draw arrows in xymatrix which stretch not over the
whole distance between to nodes, but only over  a certain percentage. I want
to use this to draw 2-cells in commutative squares.

Until now, I have done this by drawing invisible frames around source and
target nodes, as in the following example:
\[
\xymatrix{
FA
    \ar[r]^{Ff}
    \ar[d]_{\eta_A} &
FB
    \ar[d]^{\eta_B}
\\
GA
    \ar[r]_{Gf} &
GB
\\
%
\ar@{<=}"1,2"*+++\frm{};"2,1"*++\frm{}^{\eta_f}
}
\]
However, this seems overly complicated and has the disatvantage that the
invisible frames reach in all directions, and so make the entire diagram
bigger, resulting in too much space above and beneath the diagram in the
document.

Is there a easier/better way to do this?

Thanks in advance,

Jonas
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://tug.org/pipermail/xy-pic/attachments/20100826/cbf8ac33/attachment.html>