Using the `levelplot` function from the `lattice` package, how can I add a tick or line to highlight a value on the color scale next to the level plot.

More specifically, I am plotting a covariance matrix as a heatmap and for example, I want to be able to indicate the level of significance on the color scale.

```
library(lattice)

mat <- matrix(rnorm(25),ncol=5,nrow=5)
cov.m <-  cov(mat[,1:5],mat[1:5,])

levelplot(cov.m,  xlab="", ylab="", 
          col.regions= colorRampPalette(c("blue","white","red"), 
          space = "rgb"), cuts=100, at=seq(-0.2,1,0.005))

```

I now want to indicate 0.18 on the adjacent color scale.

**UPDATE**: so, I attempted to add an `abline` to `plot.trellis` and I do not get an error or a warning but it seems to be ignored. It could be that I'm not even hacking the source code correctly. Anybody with step-by-step instructions on how to modify the code in the lattice package in the context of this question?