### Name: interaction.plot
### Title: Two-way Interaction Plot
### Aliases: interaction.plot
### Keywords: hplot

### ** Examples

require(graphics)

with(ToothGrowth, {
interaction.plot(dose, supp, len, fixed=TRUE)
dose <- ordered(dose)
interaction.plot(dose, supp, len, fixed=TRUE, col = 2:3, leg.bty = "o")
interaction.plot(dose, supp, len, fixed=TRUE, col = 2:3, type = "p")
})

with(OrchardSprays, {
  interaction.plot(treatment, rowpos, decrease)
  interaction.plot(rowpos, treatment, decrease, cex.axis=0.8)
  ## order the rows by their mean effect
  rowpos <- factor(rowpos,
                   levels = sort.list(tapply(decrease, rowpos, mean)))
  interaction.plot(rowpos, treatment, decrease, col = 2:9, lty = 1)
})

with(esoph, {
  interaction.plot(agegp, alcgp, ncases/ncontrols, main = "'esoph' Data")
  interaction.plot(agegp, tobgp, ncases/ncontrols, trace.label="tobacco",
                   fixed=TRUE, xaxt = "n")
})
## deal with NAs:
esoph[66,] # second to last age group: 65-74
esophNA <- esoph; esophNA$ncases[66] <- NA
with(esophNA, {
  interaction.plot(agegp, alcgp, ncases/ncontrols, col= 2:5)
                                # doesn't show *last* group either
  interaction.plot(agegp, alcgp, ncases/ncontrols, col= 2:5, type = "b")
  ## alternative take non-NA's  {"cheating"}
  interaction.plot(agegp, alcgp, ncases/ncontrols, col= 2:5,
                   fun = function(x) mean(x, na.rm=TRUE),
                   sub = "function(x) mean(x, na.rm=TRUE)")
})
rm(esophNA) # to clear up