R: Two-way Interaction Plot

interaction.plot {base} | R Documentation

## Two-way Interaction Plot

### Description

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

### Usage

    interaction.plot(x.factor, trace.factor, response, fun = mean,
                     type = c("l", "p"), legend = TRUE,
                     trace.label = deparse(substitute(trace.factor)),
                     fixed = FALSE, xlab, ylab, ylim, lty, col = 1,
                     pch = c(1:9, 0, letters), ...)

### Arguments

`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: lines or points.
`legend` | logical. Should a legend be included?
`trace.label` | overall label for the legend.
`fixed` | 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` | the x label of the plot.
`ylab` | the y label of the plot.
`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.
`...` | graphics parameters to be passed to the plotting routines.

### Details

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 overriden). 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.

### Note

Some of the argument names and the precise behaviour are chosen for S-compatibility.

### Examples

    data(ToothGrowth)
    attach(ToothGrowth)
    interaction.plot(dose, supp, len, fixed=TRUE)
    dose <- ordered(dose)
    interaction.plot(dose, supp, len, fixed=TRUE)
    detach()

    data(OrchardSprays)
    attach(OrchardSprays)
    interaction.plot(treatment, rowpos, decrease)
    interaction.plot(rowpos, treatment, decrease)
    ## order the rows by their mean effect
    rowpos <- factor(rowpos, levels=sort.list(tapply(decrease, rowpos, mean)))
    interaction.plot(rowpos, treatment, decrease)
    detach()

    data(esoph)
    attach(esoph)
    interaction.plot(agegp, alcgp, ncases/ncontrols)
    interaction.plot(agegp, tobgp, ncases/ncontrols, trace.label="tobacco",
                     fixed=TRUE)
    detach()

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