###  Functions to accompany R-Companion

visual_test_for_association <- function(formula, data, mfrow=c(5,4), ...) {
    savePar <- par(mfrow=mfrow, mar=rep(0.5,4), cex.axis=0.1)   # squeeze tight
    on.exit(par(savePar))
    vars <- eval(attributes(terms(formula))$variables,envir=data)
    y <- vars[[1]]              # numeric vectors
    x <- vars[[2]]
    n <- nrow(data)
    r <- sample(1:mfrow[1],1)   # where to put original
    c <- sample(1:mfrow[2],1)
    for(row in 1:mfrow[1])
        for(col in 1:mfrow[2]) {
            if(row==r & col==c) plot(x,      y     , par=...)
            else                plot(x, sample(y,n), par=...)
        }
    par(savePar)
    return(c(r,c))
}

###  copied from R documentation for pairs

panel.cor <- function(x, y, digits = 2, prefix = "", cex.cor, ...)
{
    usr <- par("usr"); on.exit(par(usr))
    par(usr = c(0, 1, 0, 1))
    r <- abs(cor(x, y))
    txt <- format(c(r, 0.123456789), digits = digits)[1]
    txt <- paste0(prefix, txt)
    if(missing(cex.cor)) cex.cor <- 0.3/strwidth(txt)
    text(0.5, 0.5, txt, cex=cex.cor, col='darkgray')  # cex = cex.cor * r)
}