The method for calculating the ellipses has been modified from car::ellipse (Fox and Weisberg, 2011)

stat_ellipse(mapping = NULL, data = NULL, geom = "path",
position = "identity", ..., type = "t", level = 0.95, segments = 51,
na.rm = FALSE, show.legend = NA, inherit.aes = TRUE)

## Arguments

mapping Set of aesthetic mappings created by aes or aes_. If specified and inherit.aes = TRUE (the default), it is combined with the default mapping at the top level of the plot. You must supply mapping if there is no plot mapping. The data to be displayed in this layer. There are three options: If NULL, the default, the data is inherited from the plot data as specified in the call to ggplot. A data.frame, or other object, will override the plot data. All objects will be fortified to produce a data frame. See fortify for which variables will be created. A function will be called with a single argument, the plot data. The return value must be a data.frame., and will be used as the layer data. The geometric object to use display the data Position adjustment, either as a string, or the result of a call to a position adjustment function. other arguments passed on to layer. These are often aesthetics, used to set an aesthetic to a fixed value, like color = "red" or size = 3. They may also be parameters to the paired geom/stat. The type of ellipse. The default "t" assumes a multivariate t-distribution, and "norm" assumes a multivariate normal distribution. "euclid" draws a circle with the radius equal to level, representing the euclidean distance from the center. This ellipse probably won't appear circular unless coord_fixed() is applied. The confidence level at which to draw an ellipse (default is 0.95), or, if type="euclid", the radius of the circle to be drawn. The number of segments to be used in drawing the ellipse. If FALSE, the default, missing values are removed with a warning. If TRUE, missing values are silently removed. logical. Should this layer be included in the legends? NA, the default, includes if any aesthetics are mapped. FALSE never includes, and TRUE always includes. If FALSE, overrides the default aesthetics, rather than combining with them. This is most useful for helper functions that define both data and aesthetics and shouldn't inherit behaviour from the default plot specification, e.g. borders.

## References

John Fox and Sanford Weisberg (2011). An R Companion to Applied Regression, Second Edition. Thousand Oaks CA: Sage. URL: http://socserv.socsci.mcmaster.ca/jfox/Books/Companion

## Examples

ggplot(faithful, aes(waiting, eruptions)) +
geom_point() +
stat_ellipse()
ggplot(faithful, aes(waiting, eruptions, color = eruptions > 3)) +
geom_point() +
stat_ellipse()
ggplot(faithful, aes(waiting, eruptions, color = eruptions > 3)) +
geom_point() +
stat_ellipse(type = "norm", linetype = 2) +
stat_ellipse(type = "t")
ggplot(faithful, aes(waiting, eruptions, color = eruptions > 3)) +
geom_point() +
stat_ellipse(type = "norm", linetype = 2) +
stat_ellipse(type = "euclid", level = 3) +
coord_fixed()
ggplot(faithful, aes(waiting, eruptions, fill = eruptions > 3)) +
stat_ellipse(geom = "polygon")