getExpressionStrip.R: allows for the easy inclusion of math (expressions) in Lattice panel strips

getExpressionStrip.R

getExpressionStrip <- function(...) {
        dots<-list(...)
        if (length(dots)==0) return(strip.default)
        if (class(dots[[1]])=="list") {
                rename <- dots[[1]]
                stripparam <- dots[-1]
        } else {
                rename <- dots
                stripparam <- list()
        }
        function(...,factor.levels, var.name) {
                rn<-function(x) if (x %in% names(rename)) rename[[x]] else x;
                factor.levels<-do.call(c, lapply(factor.levels, rn))
                var.name<-do.call(c, lapply(var.name, rn))
 
                xx<-c(stripparam, list(factor.levels=factor.levels, var.name=var.name), list(...))
                return(do.call(strip.default, xx))
        }    
}

test.getExpressionStrip.R

require(lattice)

#sample data
dd<-data.frame(rsq=rep(c("alpha","betaplus1","gamma"), 4), delta=rep(c("High","Low"), each=6), x=rep(1:2, 6), y=runif(12))

#basic syntax
xyplot(y~x|rsq+delta, dd, 
    strip=getExpressionStrip(alpha=expression(alpha), betaplus1=expression(beta + 1), gamma=expression(gamma))
)

#extended syntax
xyplot(y~x|rsq+delta, dd, 
    strip=getExpressionStrip(
        list(delta=expression(Delta), rsq=expression(r^2), 
            alpha=expression(alpha), betaplus1=expression(beta+1), gamma=expression(gamma)
        ), 
        strip.names=T
    )
)