[R] adding multiple fitted curves to xyplot graph

[GOUACHE David]

Hello RHelpers,

This may already have been answered, but despite days of scouring through the archives I haven't found it.
My goal is to add multiple fitted curves to a plot.
An example data set (a data frame named df in following code) is:

             x1               y1     factor1
4       1298.25       0.00000000           1
5       1393.25       0.00000000           1
6       1471.50       0.04597701           1
7       1586.70       2.56908046           1
8       1692.10      11.14080460           1
9       1832.55      45.50459770           1
10      1928.30      65.56000000           1
11      2092.40     100.00000000           1
31      1202.90       0.00000000           2
41      1298.25       0.00000000           2
51      1393.25       0.37885057           2
61      1471.50       0.76839080           2
71      1586.70       7.75206897           2
81      1692.10      50.19448276           2
91      1832.55      94.08045977           2
101     1928.30     100.00000000           2
111     2092.40     100.00000000           2
14      1028.50       0.11111111           3
22      1106.40       0.04938272           3
32      1202.90       0.03448276           3
42      1298.25       0.34482759           3
52      1393.25       1.43850575           3
62      1471.50       1.96850575           3
72      1586.70      36.80597701           3
82      1692.10      92.83390805           3
92      1832.55     100.00000000           3
15      1028.50       0.09638554           4
23      1106.40       0.39988506           4
33      1202.90       0.49321839           4
43      1298.25       1.66045977           4
53      1393.25       7.51137931           4
63      1471.50      42.02724138           4
73      1586.70      99.12068966           4
83      1692.10     100.00000000           4

I plot this with xyplot:

trellis.par.set("background","white")
trellis.par.set(list(superpose.symbol=list(pch=c(15:17,21,25))))
xyplot(y1 ~ x1, data=df, groups=factor1,
            type = "p",
            auto.key =
            list(space = "right", points = TRUE, lines = FALSE))

For each level of factor1 I fit a growth curve:

fit.curve<-function(tab)
{
	res.fit<-nls(y1 ~ 100/(1+exp(((-log(81))/a)*(x1-b))), start=list(a=min(tab$x1[tab$y1>76],na.rm=T)-max(tab$x1[tab$y1<15],na.rm=T),b=tab$x1[abs(tab$y1-50)==min(abs(tab$y1-50),na.rm=T)][!is.na(tab$x1[abs(tab$y1-50)==min(abs(tab$y1-50),na.rm=T)])]),data=tab) 
	coef(res.fit)
}
by(df,list(df$factor1),fit.curve)

I would like to add the 4 curves corresponding to these 4 fits to my graphic.
The elegant way would be a custom panel function I suppose, but I haven't been able to write one up...
Could someone help me out on this please?

In advance thanks very much!!!

David Gouache
Arvalis - Institut du Végétal
Station de La Minière
78280 Guyancourt
Tel: 01.30.12.96.22 / Port: 06.86.08.94.32