[R] Plotting two regression lines on one graph

[David Winsemius]

Since you have provided no executable data and not even a clear enough  
description of the data to offer advice regarding approaches or  
pitfalls, I will use the example in glm's help page:

counts <- c(18,17,15,20,10,20,25,13,12)
outcome <- gl(3,1,9)
treatment <- gl(3,3)

glm.D93r <- glm(counts ~ outcome, family=poisson())
plot(x=as.numeric(as.character(outcome)), y=counts, type="p")  #need  
to convert from factor to ge points
lines(1:3, c(exp(3.045), exp(3.0445 -0.4543), exp(3.0445 -0.2930) ) )

#the default link for poisson is log, so you need to transform back to  
the original scale to predict counts.


On Jun 7, 2009, at 11:36 PM, Jo Stringer wrote:
>
> I have fitted two glms assuming a poisson distribution which are:
>
> fit1 <- glm(Aids ~ Year, data=aids, family=poisson())
>
> fit2 <- glm(Aids ~ Year+I(Year^2), data=aids, family=poisson())
>
>
> I am trying to work out how to represent the fitted regression  
> curves of fit1 and fit2 on the one graph. I have tried:
>
> graphics.off()
>
> plot(Aids ~ Year, data = aids)
>
>
> line(glm(Aids ~ Year, data=aids, family=poisson()))
> line(glm(Aids ~ Year+I(Year^2), data=aids, family=poisson()))
>
> but this does not work.
>
> Can anyone help me?


David Winsemius, MD
Heritage Laboratories
West Hartford, CT