# Ellipses

From car v0.8-3
by John Fox

##### Ellipses, Data Ellipses, and Confidence Ellipses

These functions draw ellipses, including data ellipses, and confidence ellipses for linear and generalized linear models.

##### Usage

```
ellipse(center, shape, radius, center.pch=19, center.cex=1.5,
segments=51, add=T, xlab="", ylab="",
las=1, col=palette()[2], lwd=2, lty=1, ...)
data.ellipse(x, y, levels=c(0.5, 0.9), center.pch=19, center.cex=1.5,
plot.points=T, add=!plot.points, segments=51, robust=F,
xlab=deparse(substitute(x)),
ylab=deparse(substitute(y)),
las=1, col=palette()[2], pch=1, lwd=2, lty=1, ...)
confidence.ellipse(model, ...)
confidence.ellipse.lm(model, which.coef, levels=0.95, Scheffe=F,
center.pch=19, center.cex=1.5, segments=51, xlab, ylab,
las=1, col=palette()[2], lwd=2, lty=1, ...)
confidence.ellipse.glm(model, which.coef, levels=0.95, Scheffe=F,
center.pch=19, center.cex=1.5, segments=51, xlab, ylab,
las=1, col=palette()[2], lwd=2, lty=1, ...)
```

##### Arguments

- center
- 2-element vector with coordinates of center of ellipse.
- shape
- $2\times 2$ shape (or covariance) matrix.
- radius
- radius of circle generating the ellipse.
- center.pch
- character for plotting ellipse center.
- center.cex
- relative size of character for plotting ellipse center.
- segments
- number of line-segments used to draw ellipse.
- add
- if
`TRUE`

add ellipse to current plot. - xlab
- label for horizontal axis.
- ylab
- label for vertical axis.
- x
- a numeric vector, or (if
`y`

is missing) a 2-column numeric matrix. - y
- a numeric vector, of the same length as
`x`

. - plot.points
- if
`FALSE`

data ellipses are added to the current scatterplot, but points are not plotted. - levels
- draw elliptical contours at these (normal) probability or confidence levels.
- robust
- if
`TRUE`

use the`cov.trob`

function in the`MASS`

package to calculate the center and covariance matrix for the data ellipse. - model
- a model object produced by
`lm`

or`glm`

. - which.coef
- 2-element vector giving indices of coefficients to plot; if missing, the first two coefficients (disregarding the regression constant) will be selected.
- Scheffe
- if
`TRUE`

scale the ellipse so that its projections onto the axes give Scheffe confidence intervals for the coefficients. - las
- if
`0`

, ticks labels are drawn parallel to the axis; set to`1`

(the default) for horizontal labels (see`par`

). - col
- color for points and lines; the default is the
*second*entry in the current color palette (see`palette`

and`par`

). - pch
- plotting character for points; default is
`1`

(a circle, see`par`

). - lwd
- line width; default is
`2`

(see`par`

). - lty
- line type; default is
`1`

, a solid line (see`par`

). - ...
- other plotting parameters to be passed to
`plot`

and`line`

.

##### Details

The ellipse is computed by suitably transforming a unit circle.
`data.ellipse`

superimposes the normal-probability contours over a scatterplot
of the data.

##### Value

`NULL`

. These functions are used for their side effect: producing plots.

##### References

Fox, J. (1997)
*Applied Regression, Linear Models, and Related Methods.* Sage.
Monette, G. (1990)
Geometry of multiple regression and 3D graphics.
In Fox, J. and Long, J. S. (Eds.)
*Modern Methods of Data Analysis.* Sage.

##### See Also

##### Examples

```
data(Prestige)
attach(Prestige)
data.ellipse(income, education, levels=0.1*1:9, lty=2)
confidence.ellipse(lm(prestige~income+education), Scheffe=T)
```

*Documentation reproduced from package car, version 0.8-3, License: GPL version 2 or newer*

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