[R] Problem plotting with xyplot

[Benjamin Tyner]

This is in regard to Dan Norlund's post from December 2003 (available at 
https://stat.ethz.ch/pipermail/r-help/attachments/20031225/1bce02b8/attachment.pl 
)

While I agree with Prof. Ripley's warning against fitting so few 
observations per panel, this is not actually what causes the error. 
Instead, the culprit is the fact that panel.loess uses 
family='symmetric' by default; since the first iteration of fitting 
interpolates the data, the residuals are zero, hence the robustness 
weights are not defined for the next iteration. In the example Dan gave, 
a change to

xyplot(tolerance~age | factor(id), data=tolerance.pp, 
type=c("p","smooth"), family="gaussian")

avoids this. As family='gaussian' is the default outside of panel.loess, 
it is easy to forget this detail.

Ben