ctree ignores variables with non-synchronic names?

Thanks for pointing this out. This was indeed a bug in partykit::ctree but has been fixed now in version 1.2-11 (the current development version on R-Forge).

Furthermore, if you just want the non-syntactic label to be used in printing and plotting you can use the following quick & dirty workaround: First learn the data with nice syntactic names.

myData <- data.frame(
  Y = factor(rep(LETTERS[1:3], each = 10)),
  x1 = 1:30,
  x2 = c(1:10, 2:11, 3:12)
)
ct <- ctree(Y ~ ., data = myData)

then after fitting the tree, change the name of the variable in the $data stored in the tree.

names(ct$data)[2] <- "x 1"

This is then used in printing and plotting.

print(ct)
## Model formula:
## Y ~ x1 + x2
## 
## Fitted party:
## [1] root
## |   [2] x 1 <= 10: A (n = 10, err = 0.0%)
## |   [3] x 1 > 10
## |   |   [4] x 1 <= 20: B (n = 10, err = 0.0%)
## |   |   [5] x 1 > 20: C (n = 10, err = 0.0%)
plot(ct)