Accessing data

[Jindan Zhou <jindan@jindan.homedns.org>]

With help from the list, my dataset looks neater now, but I still need
help:


  SiteID Summ.Per. Year  NO3      Dates          X Latitude Longitude
1   AL02    Summer 2001 0.86   6/5/2001  8/28/2001  30.7905  -87.8497
2   AL02      Fall 2001 0.47  8/28/2001 11/27/2001  30.7905  -87.8497
3   AL02    Winter 2002 0.62 11/27/2001  2/26/2002  30.7905  -87.8497
....
....
17675   WY99      Fall 2003 1.22  9/2/2003 12/2/2003  43.8733 -104.1922
17676   WY99    Winter 2004 1.22 12/2/2003  3/2/2004  43.8733 -104.1922

If I need to manipulate data for a particular season, say, simple
plots of NO3 for Summer/1995 for all sites, what is the S code to call those
data?

Thanks for any input,

Jindan