Fitting confidence interval lines in R using the predict function

Hi everyone,

I am currently attempting to fit confidence interval lines to a regression model in R, but every time I try to for example add a +2 confidence interval line, I end up with two of them! I have been over my code many times and can't find any obvious mistakes. This is the code I have used:




Here is some information about girth, which is the explanatory variable causing the problem - all the other explanatory variables work fine using this code and produce the correct number of lines.

> summary(girth)
Min. 1st Qu. Median Mean 3rd Qu. Max. NA's
38.00 70.00 80.00 79.83 88.00 112.00 5

Does anyone know where I might have gone wrong? I would really appreciate the help! :)