car (version 0.8-1)

Ellipses: Ellipses, Data Ellipses, and Confidence Ellipses

Description

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.

Value

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

Details

The ellipse is computed by suitably transforming a unit circle. data.ellipse superimposes the normal-probability contours over a scatterplot of the data.

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

cov.trob.

Examples

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