I don't know what SigmaPlot and Excel are doing for you, but I would guess that they are not doing cubic splines (as a general rule, when R and Excel differ, it is safest to assume that R is not the one doing something wrong)

Often differences between packages are due to differences in assumptions or model specifications. I don't know SigmaPlot to be able to say what it is doing and a quick search of the excel help for 'spline' was not enlightening.

Perhaps what you are looking for is not a cubic spline. Another option is an xspline. These are implemented in the grid package. Try:

library(grid)

library(lattice)

x<-c(-45,67,131,259,347)

y <- c(0.31, 0.45, 0.84, 0.43, 0.25)

s <- seq(-1,1, .25)

tmp.df <- data.frame(x=rep(x,9), y=rep(y,9), s=rep(s, each=5))

xyplot(y~x|factor(s), data=tmp.df, tmp.s=tmp.df$s, panel=function(x,y,subscripts,tmp.s,...){

grid.points(x,y)

tmp2.s <- tmp.s[subscripts]

print(tmp2.s)

grid.xspline(x,y, default.units='native', shape=tmp2.s)

})

Hope this helps,

--

Gregory (Greg) L. Snow Ph.D.

Statistical Data Center

Intermountain Healthcare

[hidden email]
(801) 408-8111

> -----Original Message-----

> From: Nestor Fernandez [mailto:

[hidden email]]

> Sent: Thursday, September 20, 2007 4:02 AM

> To:

[hidden email]
> Cc: Greg Snow

> Subject: Re: [R] Smooth line in graph

>

> Sorry, I answered too quickly.

> It worked with the "simplified" example I provided but not

> with non-regular intervals in x:

>

> x<-c(-45,67,131,259,347)

> y <- c(0.31, 0.45, 0.84, 0.43, 0.25)

>

> plot(x,y)

> lines(spline(x,y, method='n', n=250))

> #or:

> lines(predict(interpSpline(x, y)))

>

> Produce the same decrease between first two points and the

> shape is quite different to that produced by sigmaplot -I

> need comparable figures. Changing "method" and "n" arguments

> did not help.

>

> Sorry for bothering. Any other suggestion?

>

>

>

>

>

>

> Greg Snow escribió:

> > Try:

> >

> >

> >> lines(spline(x,y, method='n', n=250))

> >>

> >

> >

>

>

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