R help - Oct 2003 - plot with dates on x axis, how to fix the number of days betwenn tick marks ?

Hi,

Following plot is displaying fine, (starting arount the 10. september),
except that the xaxp parameter has no effect.

I'd like to have a tickmark every 7 days...

plot(timeline,
   subset(myd, TYPE=="A")$list1,
   ylim=c(100*floor(min(subset(myd, TYPE=="A")$list1)/100-1),
        100*ceiling(max(subset(myd, TYPE=="A")$list1)/100+1)),
   xlim=c(1063500000, Sys.time()),
   xaxp=c(1063500000, Sys.time(),7*24*3600),
   type="o", col="blue",ylab="")


Thanks for your hints.


Marc Mamin