# 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 used for its side-effect of drawing labels on a plot.
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 2.0-11, License: GPL (>= 2)*