R for-loop to add layer to lattice plot

Hello,
I am using e1071 to run support vector machine. I would like to plot
the data with lattice and specifically show the hyperplanes created by
the system.
I can store the hyperplane as a contour in an object, and I can plot
one object at a time. Since there will be thousands of elements to
plot, I can't manually add them one by one to the plot, so I tried to
loop into them, but only the last is added.
Here it the working example for more clarity:

```
library(e1071)
library(lattice)
library(latticeExtra)

make.grid <- function(x, n = 1000) {
  grange = apply(x, 2, range)
  x1 = seq(from = grange[1,1], to = grange[2,1], length = n)
  x2 = seq(from = grange[1,2], to = grange[2,2], length = n)
  expand.grid(X1 = x1, X2 = x2)
}

plot_list <- list()
for (i in 1:10) {
  x1 = rnorm(100, mean = 0.2, sd = 0.15)
  y1 = rnorm(100, mean = 0.7, sd = 0.15)
  y2 = rnorm(100, mean = 0.2, sd = 0.15)
  x2 = rnorm(100, mean = 0.75, sd = 0.15)
  df = data.frame(x = c(x1,x2), y=c(y1,y2),
                  z=c(rep(0, length(x1)), rep(1, length(x2))))
  df$z = factor(c(rep(0, length(x1)), rep(1, length(x2))))
  df[, "train"] <- ifelse(runif(nrow(df)) < 0.8, 1, 0)
  trainset <- df[df$train == 1, ]
  testset <- df[df$train == 0, ]
  trainColNum <- grep("train", names(df))
  trainset <- trainset[, -trainColNum]
  testset <- testset[, -trainColNum]
  svm_model <- svm(z ~ .,
                  data = trainset,
                  type = "C-classification",
                  kernel = "linear",
                  scale = FALSE)
  # generate contour
  xmat = make.grid(matrix(c(testset$x, testset$y),
                          ncol = 2, byrow=FALSE))
  xgrid = as.data.frame(xmat)
  names(xgrid) = c("x", "y")
  z = predict(svm_model, xgrid)
  xyz_dat = as.data.frame(cbind(xgrid, z))
  plot_list[[i]] = contourplot(z ~ y+x, data=xyz_dat, pretty = TRUE,
                               xlim=c(-1,50), ylim=c(-0.001, 0.05),
                               labels = FALSE, col = "blue", lwd = 0.5)

}
# the contour is stored in the object plot_list
str(plot_list) # confirm that there is data here

# I can add one element at the time to lattice's xyplot and store it
in an object P
P = xyplot(y ~ x, group = z, data = df,
           pch = 16, cex = 1.5, alpha = 0.25) + as.layer(plot_list[[1]]) +
  as.layer(plot_list[[2]])
plot(P)  # this demonstrates that the lines are not the same

# but if I add the elements via loop, it does not work
for (i in 1:length(plot_list)) {
  print(i)
  P = xyplot(y ~ x, group = z, data = df,
             pch = 16, cex = 1.5, alpha = 0.25) + as.layer(plot_list[[i]])
}
plot(P)
```

Am I missing something?

Thank you