[R] How to use panel.qqmathline?

[cjosephlu@seed.net.tw]

Dear R users:

How can I use panel.qqmathline, in package lattice, to add
straight lines onto the plots generated by qqmath?
I read help pages of qqmath, panel.qqmathline, xyplot, ...,
but just can't one example that shows how to make it work.

For example, 

> data(sleep)
> qqnorm(~ extra | group, data=sleep, aspect=1)

how can I use panel.qqmathline?

Thanks very much for your help,

Joseph Lu
Department of Statistics
National Cheng-Kung University
Tainan, Taiwan

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