[R] Bivariate Scatter Plots with Lattice

[Rich Shepard]

   Data frame has this structure:

'data.frame':	11169 obs. of  4 variables:
  $ stream  : Factor w/ 37 levels "Burns","CIL",..: 1 1 1 1 1 1 1 1 1 1 ...
  $ sampdate: Date, format: "1987-07-23" "1987-09-17" ...
  $ param   : Factor w/ 8 levels "As","Ca","Cl",..: 1 1 1 1 1 1 1 1 1 1 ...
  $ quant   : num  0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0 ...

   I want to create scatter plots of the concentrations (numeric column
'quant') for two specified chemicals (factor column 'param') for all dates
and streams (factor column 'stream') in which there are data.

   Looking in ?lattice and Deepayan's book did not provide an example. I've
tried varies formulae but without success, and would appreciate a pointer to
documentation and examples of how to write such an expression.

Rich