# showLabels

##### Utility Functions to Identify and Mark Extreme Points in a 2D Plot.

This function is called by several graphical functions in the `car`

package to mark extreme points in a 2D plot. Although the user is unlikely
to call this function directly, the documentation below applies to all
these other functions.

- Keywords
- utilities

##### Usage

```
showLabels(x, y, labels=NULL, id.method="identify",
id.n = length(x), id.cex=1, id.col=palette()[1], ...)
```

##### Arguments

- x
- Plotted horizontal coordinates.
- y
- Plotted vertical coordinates.
- labels
- Plotting labels. If
`NULL`

, case numbers will be used. If labels are long, the`substr`

or`abbreviate`

function can be used to shorten them. - id.method
- How points are to be identified. See Details below.
- id.n
- Number of points to be identified. If set to zero, no points are identified.
- id.cex
- Controls the size of the plotted labels. The default is
`1`

. - id.col
- Controls the color of the plotted labels.
- ...
- additional arguments passed to
`identify`

or to`text`

.

##### Details

The argument `id.method`

determine how the points
to be identified are selected. For the default value of `id.method="identify"`

,
the `identify`

function is used to identify points
interactively using the mouse. Up to `id.n`

points can be identified,
so if `id.n=0`

, which is the default in many functions in the `car`

package, then no point identification is done.
Automatic point identification can be done depending on the value of the
argument `id.method`

.

`id.method = "x"`

select points according to their value of`abs(x - mean(x))`

`id.method = "y"`

select points according to their value of`abs(y - mean(y))`

`id.method = "mahal"`

Treat`(x, y)`

as if it were a bivariate sample, and select cases according to their Mahalanobis distance from`(mean(x), mean(y))`

`id.method`

can be a vector of the same length as`x`

consisting of values to determine the points to be labeled. For example, for a linear model`m`

, setting`id.method=cooks.distance(m), id.n=4`

will label the points corresponding to the four largest values of Cook's distance, or`id.method = abs(residuals(m, type="pearson")), id.n=2`

would label the two observations corresponding to the largest absolute Pearson residuals.`id.method`

can be a vector of case numbers or case-labels, in which case those cases will be labeled, as long as`id.n`

is greater than zero.

`showLabels`

, the `id.method`

argument can be list, so, for
example `id.method=list("x", "y")`

would label according to the horizontal
and vertical axes variables.
Finally, if the axes in the graph are logged, the function uses logged-variables
where appropriate.
##### Value

- A utility function primarily used for its side-effect of drawing labels on a plot. Returns invisibly the labels of the selected points, or NULL if no points are selected. Although intended for use with other functions in the car package, this function can be used directly.

##### References

Fox, J. and Weisberg, S. (2011) *An R Companion to Applied Regression*,
Second Edition, Sage.
Weisberg, S. (2005) *Applied Linear Regression*, Third Edition, Wiley.

##### See Also

##### Examples

```
plot(income ~ education, Prestige)
with(Prestige, showLabels(education, income,
labels = rownames(Prestige), id.method=list("x", "y"), id.n=3))
m <- lm(income ~ education, Prestige)
plot(income ~ education, Prestige)
abline(m)
with(Prestige, showLabels(education, income,
labels=rownames(Prestige), id.method=abs(residuals(m)), id.n=4))
```

*Documentation reproduced from package car, version 2.0-13, License: GPL (>= 2)*