xyplot time series with positive values in green, negative in red, in R

Question

Is there a neat way to color negative values in red and others in green for a (simplified) time series plot below, using lattice::xyplot?

set.seed(0)
xyplot(zoo(cumsum(rnorm(100))), grid=T)

Answer 1

Lattice is based on grid so you can use grid's clipping functionality

library(lattice)
library(grid)

set.seed(0)
x <- zoo(cumsum(rnorm(100)))

xyplot(x, grid=TRUE, panel = function(x, y, ...){
       panel.xyplot(x, y, col="red", ...) 
       grid.clip(y=unit(0,"native"),just=c("bottom"))
       panel.xyplot(x, y, col="green", ...) })