[R] xyplot() or splom()?: two factors from same data frame

[Rich Shepard]

   I'm not seeing how to plot the quantities associated with two values of a
factor by reading the ?xyplot help page and Deepayan's book. Perhaps I need
to generate many different subsets from the data frame, but that would
require _many_ new data frames.

   The structure of the data frame (for a single stream) has sites (factor),
a date (as.Date), parameters (factor), and quantities (numeric). What I need
to do is produce scatter plots of the quantities associated with two
different parameters conditioned by site or by date (separate plots, I'm
sure).

   I have 24 streams and initially need to look at the relationships of 5
different pairs of parameters (e.g., "Cond" and "TDS") for all sites and by
sites. Can I do this from the existing data frame?

   It seems to me that a splom() could be ideal for this, but I still have
the question of how to specify quantities for two parameters to plot against
each other.

Rich