[R] mosaic.by(): vectorizing args passed by apply()?

[Michael Friendly]

I've just started learning R, so I'm still on the steep part of the
learning curve, but my enthusiasm was heightened by learning that
there's a very nice implementation of mosaicplot().

As a learning project, I've already done a basic implementation
of a pairs.table() function which does a mosaic scatterplot matrix,
and now I'm trying to do conditional mosaic plots (discrete analog
of a coplot).

I found that
   apply(table, by, mosaicplot,...)
worked quite nicely, but I want to label each mosaic with the
combination of the factor name and level value, e.g., Sex: Male,
and I can't figure out how to get just one element of a list
passed by apply as an argument.  

In the function below, titles is a list like:
> paste("Sex: ", dimnames(Titanic)[[2]])
[1] "Sex:  Male"   "Sex:  Female"
and I want each element passed as the main= value for the corresponding
slice of the table.
Can someone help?

Try the function below with

data(HairEyeColor)
mosaic.by(HairEyeColor, 3)

## Conditional mosaics, one for each level of the by-variable(s) 
##   -- how to add factor levels as main= ?
##   -- how to do for more than one by-variable?
mosaic.by <- 
function(table, by=NULL, ...)
{
    n <- length(dim(table))
    if(n == 0)
        stop("invalid table `table'")
    if(length(by)>1)
        stop("Sorry, cannot handle >1 by variable yet.")
        
    np <- dim(table)[by]
    name <- names(dimnames(table))[by]
    titles <- paste(name, ":", dimnames(table)[[by]])
    
    opar <- par( mfrow=c(np, np), usr=c(1,1000,1,1000), mar=rep(1/2,4))
    on.exit(par(opar))
    
    apply(table, by, mosaicplot, main=titles, shade=T, legend=F, ...)    
}

thx,
-Michael
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