[R] Linear Trend in Residiuals From lme

[Rick Bilonick]

I'm fitting a mixed effects model:

fit.1 <- lme(y~x,random=~1|id,data=df)

There are two different observations for each id for both x and y. When
I use plot(fit.1), there is a strong increasing linear trend in the
residuals versus the fitted values (with no outliers). This also happens
if I use random=~x|id. Am I specifying something incorrectly?

Rick B.