### Interaction plot

An interaction plot is a visual representation of the interaction between the effects of two factors, or between a factor and a numeric variable. It is suitable for experimental data.

You can create an interaction plot with the `interaction.plot`

function. The command takes the general form:

> interaction.plot(`dataset`

$`var1`

,`dataset`

$`var2`

,`dataset`

$`response`

)

where `var1`

and `var2`

are the names of the explanatory variables and `response`

is the name of the response variable.

If one of the explanatory variables is numeric and the other is a factor, list the numeric variable first and the factor second. This way the numeric variable is displayed along the x-axis and the factor is represented by separate lines on the plot.

## Example: Interaction plot with `ToothGrowth`

data

Consider the `ToothGrowth`

dataset, which is included with R. The dataset gives the results of an experiment to determine the effect of two supplements (Vitamin C and Orange Juice), each at three different doses (0.5, 1 or 2 mg) on tooth length in guinea pigs. The `len`

variable gives the tooth growth, the `supp`

variable gives the supplement type and the `dose`

variable gives the supplement dose. You can view more information about the `ToothGrowth`

dataset by entering `help(ToothGrowth)`

.

To create an interaction plot illustrating the interaction between supplement type and supplement dose, use the command:

> interaction.plot(ToothGrowth$dose, ToothGrowth$supp, ToothGrowth$len)

The results are shown below.

Interaction plot for the ToothGrowth data