sigmoid <-
function(x, y,...){
panel.barchart(x, y, ...)
panel.curve( exp(m*x + k)/(1 + exp(m*x+k)), add=T, lwd=2)
panel.abline(h = .5, col="slategray", lwd=2, lty=2)
panel.abline(v = -k/m, col="slategray", lwd=2, lty=3)
}