将图例添加到R中的constparty树图中

Question

任何人都可以帮我在二叉树的情节中添加一个图例吗？每次我尝试时都会收到错误消息。见下面的例子：

library(partykit)

irisct <- ctree(Species ~ .,data = iris)
irisct
plot(irisct)

legend("topleft", legend = c("Label 1", "Label 2", "Label 3"), fill = c("#3f3f3f", "#7e7e7e", "#a8a8a8"))

当我这样做时，图例不会出现，我收到以下消息：

Error: invalid graphics state

当我致电sessionInfo()时，我得到以下输出：

R version 3.0.2 (2013-09-25)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252    LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C                           LC_TIME=English_United States.1252    

attached base packages:
[1] grid      stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] Formula_1.1-1  partykit_0.1-6

loaded via a namespace (and not attached):
[1] rJava_0.9-6        RWeka_0.4-21       RWekajars_3.7.10-1 splines_3.0.2      survival_2.37-4    tools_3.0.2   

请注意，我已按照此处的讨论尝试重新安装ggp​​lot2：Persistent invalid graphics state error when using ggplot2

任何建议都表示赞赏。非常感谢!!