[pstricks] Problem with y-Label

[Denis Vergès]

Hello,
This code :
%%%%%%%%%%%
\psset{xunit=1.25cm,yunit=0.01cm}
\begin{pspicture}(0,5100)(7,5900)
\psaxes[linewidth=1.5pt,Oy=5100,Dy=100](0,5100)(7,5800)
\uput[u](3.5,5850){Vente de pots de plantes}
\rput{90}(-0.5,0){nombre de pots en milliers}
\uput[d](3.5,5050){ann\'ee}
\psdots[dotstyle=square*](1,5702)(2,5490)(3,5400)(4,5319)(5,5200)(6,5180 
)
\multido{\n=5100+100}{8}{\psline(0,\n)(7,\n)}
\end{pspicture}
%%%%%%%%%%%%%%%%%%%%%%%%%
gives me the y-Label : 5100, 5200, 5300, 5400, 00, 5600, 5700, 5800.
Why 00 instead of 5500 ?
Thanks for all
(Dy=50 gives the same error)
Denis