[R] Interaction plot between 2 continuous variables

[Eiko Fried]

I have two very strong fixed effects in a LMM (both continuous variables).
model <- lmer( y ~ time + x1+x2 + (time|subject))

Once I fit an interaction of these variables, both main effects
disappear and I get a strong interaction effect.
model <- lmer( y ~ time + x1*x2 + (time|subject))

I would like to plot this effect now, but have not been able to do so,
reading through ggplot2 and lattice tutorials.

Any help would be greatly appreciated. Thank you!