I was unable to run your code; 'fac' is missing and npk$fac in teh
interaction.plot returns NA.
> data(npk, package="MASS")
> fit <- by(npk, npk$block, function(bydata) fitted.values(lm(yield ~ N,
> data=bydata)))
> fit <- unlist(fit)
> interaction.plot(npk$N, npk$block, fit, xlab="N",
> ylab="yield") # fake factor, numeric fac <-
> c(rep(1,12),rep(2,12)) # plots everything in black
> interaction.plot(npk$N, npk$block, fit, xlab="N",
> ylab="yield",col=fac)
> cbind(npk$block,fac)
> # plots everything transparent
> fac <- as.factor(fac)
> interaction.plot(npk$N, npk$block, fit, xlab="N",
> ylab="yield",col=as.numeric(npk$fac))
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