Lattice panel function to fit and draw a confidence ellipsoid from bivariate data.

A lattice panel function that computes and draws a confidence ellipsoid from bivariate data, possibly grouped by a third variable.

panel.ellipse(x, y, groups = NULL,
              level = 0.68, segments = 50, robust = FALSE,
              center.pch = 3, center.cex = 2, ...,
              type, pch, cex)

Arguments

x, y: Numeric vectors of same length giving the bivariate data. Non-numeric variables will be coerced to be numeric.
groups: Optional grouping variable.
level: Confidence level for the ellipse.
segments: Number of segments used to approximate the ellipse.
robust: Logical indicating whether a robust method should be used. If TRUE, the confidence ellipse is based on a bivariate t-distribution using the cov.trob function in the MASS package.
center.pch: Plotting character for the center (fitted mean). If NULL, the center will not be shown on the plot.
center.cex: Character expansion (size) multipler for the symbol indicating the center.
...: Further arguments, typically graphical parameters. Passed on to panel.xyplot.
type, pch, cex: Parameters that are ignored; these are present only to make sure they are not inadvertently passed on to panel.xyplot.

Author

Deepayan Sarkar, extending code contributed by Michael Friendly.

Examples


xyplot(Sepal.Length ~ Petal.Length, groups=Species,
       data = iris, scales = "free",
       par.settings = list(superpose.symbol = list(pch=c(15:17)),
                           superpose.line = list(lwd=2, lty=1:3)),
       panel = function(x, y, ...) {
           panel.xyplot(x, y, ...)
           panel.ellipse(x, y, ...)
       },
       auto.key = list(x = .1, y = .8, corner = c(0, 0)))


## Without groups
xyplot(Sepal.Length ~ Petal.Length, 
       data = iris, scales = "free",
       par.settings = list(plot.symbol = list(cex = 1.1, pch=16)),
       panel = function(x, y, ...) {
           panel.xyplot(x, y, ...)
           panel.ellipse(x, y, lwd = 2, ...)
       },
       auto.key = list(x = .1, y = .8, corner = c(0, 0)))



## With conditioning
xyplot(Sepal.Length ~ Petal.Length | Species, 
       data = iris, scales = "free",
       par.settings = list(plot.symbol = list(cex = 1.1, pch=16)),
       layout=c(2,2),
       panel = function(x, y, ...) {
           panel.xyplot(x, y, ...)
           panel.ellipse(x, y, lwd = 2, ...)
       },
       auto.key = list(x = .6, y = .8, corner = c(0, 0)))



## Compare classical with robust 
xyplot(Sepal.Length ~ Petal.Length | Species, 
       data = iris, scales = "free",
       par.settings = list(plot.symbol = list(cex = 1.1, pch=16)),
       layout=c(2,2),
       panel = function(x, y, ...) {
           panel.xyplot(x, y, ...)
           panel.ellipse(x, y, lwd = 2, ...)
           panel.ellipse(x, y, lwd = 2, col="red", robust=TRUE,  ...)
       })