[R] a question on plotting nonlinear regression

[Mark Seeto]

Mark Seeto wrote:
> 
> 
> garciap wrote:
>> 
>> 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$PI, fitted(regression))
>> 
>> but the plotted line is not the quadratic one, rather it appears a number
>> of lines whose meaning I do not know. It seems to be an stupid trouble,
>> but I tried a number of alternatives and nothing works.
>> 
>> Regards,
>> 
>> pablo
>> 
> 
> The problem could be that cubs$PI is not sorted. You could try
> 
> plot(Survival~PI,data=cubs, ylab="Survival", xlab="PI")
> lines(sort(PI), fitted(regression)[order(cubs$PI)]) 
> 
> Also, you could use linear regression with lm instead of using nls.
> 
> Regards,
> Mark
> 

Sorry, that should have been
plot(Survival~PI,data=cubs, ylab="Survival", xlab="PI") 
lines(sort(cubs$PI), fitted(regression)[order(cubs$PI)]) 


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