Plots the mean (or other summary) of the response for two-way combinations of factors, thereby illustrating possible interactions.

```
interaction.plot(x.factor, trace.factor, response, fun = mean,
type = c("l", "p", "b", "o", "c"), legend = TRUE,
trace.label = deparse(substitute(trace.factor)),
fixed = FALSE,
xlab = deparse(substitute(x.factor)),
ylab = ylabel,
ylim = range(cells, na.rm = TRUE),
lty = nc:1, col = 1, pch = c(1:9, 0, letters),
xpd = NULL, leg.bg = par("bg"), leg.bty = "n",
xtick = FALSE, xaxt = par("xaxt"), axes = TRUE,
…)
```

x.factor

a factor whose levels will form the x axis.

trace.factor

another factor whose levels will form the traces.

response

a numeric variable giving the response

fun

the function to compute the summary. Should return a single real value.

type

the type of plot (see `plot.default`

): lines
or points or both.

legend

logical. Should a legend be included?

trace.label

overall label for the legend.

fixed

logical. Should the legend be in the order of the levels
of `trace.factor`

or in the order of the traces at their right-hand ends?

xlab,ylab

the x and y label of the plot each with a sensible default.

ylim

numeric of length 2 giving the y limits for the plot.

lty

line type for the lines drawn, with sensible default.

col

the color to be used for plotting.

pch

a vector of plotting symbols or characters, with sensible default.

xpd

leg.bg, leg.bty

arguments passed to `legend()`

.

xtick

logical. Should tick marks be used on the x axis?

xaxt, axes, …

graphics parameters to be passed to the plotting routines.

By default the levels of `x.factor`

are plotted on the x axis in
their given order, with extra space left at the right for the legend
(if specified). If `x.factor`

is an ordered factor and the levels
are numeric, these numeric values are used for the x axis.

The response and hence its summary can contain missing values. If so, the missing values and the line segments joining them are omitted from the plot (and this can be somewhat disconcerting).

The graphics parameters `xlab`

, `ylab`

, `ylim`

,
`lty`

, `col`

and `pch`

are given suitable defaults
(and `xlim`

and `xaxs`

are set and cannot be overridden).
The defaults are to cycle through the line types, use the foreground
colour, and to use the symbols 1:9, 0, and the capital letters to plot
the traces.

Chambers, J. M., Freeny, A and Heiberger, R. M. (1992)
*Analysis of variance; designed experiments.*
Chapter 5 of *Statistical Models in S*
eds J. M. Chambers and T. J. Hastie, Wadsworth & Brooks/Cole.

# NOT RUN { 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 # }