Two things I'd try:

> run your code in a clean R session, started at the command line with R
> --vanilla and just loading that exact dataset and running that line of
> code, in case there are issues with other things in your session.
>
> Upgrade to the current version of R.
>
>
Dear Sarah,

I upgraded to the new R and tried R --vanilla but I still see no lines. In
the legend,as earlier,I see a line through a point but in the graph I see
only points. No lines. Any ideas now ?

> sessionInfo()
R version 2.13.0 (2011-04-13)
Platform: i486-pc-linux-gnu (32-bit)

locale,
 [1] LC_CTYPE=en_IN       LC_NUMERIC=C         LC_TIME=en_IN
 [4] LC_COLLATE=en_IN     LC_MONETARY=C        LC_MESSAGES=en_IN
 [7] LC_PAPER=en_IN       LC_NAME=C            LC_ADDRESS=C
[10] LC_TELEPHONE=C       LC_MEASUREMENT=en_IN LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base
>

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