bug in saving figure with dashed lines

Hi,
I just produced a plot with a curve dashed line:

when saving it into a png file it works fine.
But when saving it in a postscript the dashed line looks solid on most
of the plot.
This happens if there are too many points in the line:

x = arange(0,1.,0.0001)
y = sqrt(x)
plot(x,y,'k--')
## Png file is ok
savefig("toto.png")
## Eps file is NOT ok
savefig("toto.eps")

Any cure to that behaviour?

Thanks!!!
Eric