centroid based ellipse

Hello
After performing a pca of morphometric variables and a scatterplot of
the first 2 principal components, using the package latticeExtra, I
tried to draw the ellipses representing one sigma from the centroid
for each sample, but the function panel.ellipse draws the ellipses
around 95% of the sample.
Does anyone know a function to draw a centroid based ellipse?

Cheers,
Lumila
--
Lumila Paula Men?ndez
Licenciada en Antropolog?a
Divisi?n Antropolog?a-Museo de La Plata.
Universidad Nacional de La Plata.
http://www.evomorfologica.com.ar/
http://unlp.academia.edu/LumilaPm