[R] xyplot(): How to Apply Conditional?

[Rich Shepard]

On Wed, 17 Oct 2012, David Winsemius wrote:

> What exactly is being requested? Do you just need to learn how to use the
> subset parameter? I would not think that the conditioning syntax would be
> used when there were only one level desired.

David,

   When I try

xyplot(quant ~ sampdate | era, data = sb.d, main = 'Antimony Concentrations
by Time', ylab = 'Concentration (mg/L)', xlab = 'Time',
subset=era('Pre-Mining'))

this error is returned:

Error in eval(expr, envir, enclos) : could not find function "era"

But, 'era' is a factor in the data frame.

   The syntax above works with bwplot() and I would like to understand why
it's not working with xyplot().

Thanks,

Rich