[R] a question on plotting nonlinear regression

[garciap]

Hi to all the people,

I'm having a trouble when trying to plot a quadratic function. I have the
code:

regression<-nls(Survival~beta1+beta2*PI+beta3*PI^2, data=cubs,
start=list(beta1 = 1, beta2 = 1, beta3 = 1))
plot(Survival~PI,data=cubs, ylab="Survival", xlab="PI")
lines(cubs$PI, fitted(regression))

but the plotted line is not the quadratic one, rather it appears a number of
lines whose meaning I do not know. It seems to be an stupid trouble, but I
tried a number of alternatives and nothing works.

Regards,

pablo

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