Unable to add Greek/Math/Expression Split labels using rpart.plot - r

Question

I'm attempting to plot an rpart tree where I'd like to change some of the split labels to their greek/math equivalent. For instance, I have a column named mu -- I'd like this to show up as the greek letter $\mu$.
Unfortunately, when I replace one of the labels, it results in the error "Error in strsplit(labs, "\n\n") : non-character argument". As I'm not using strsplit, this error must be coming from rpart.plot call where it is assuming the labels are all plain text. This is my code:
split.fun <- function(x, labs, digits, varlen, faclen)
{
for(i in 1:length(labs)) {
if(substring(labs[i],0,2)=="mu"){
#labs[i] <- bquote(mu ~ .(substring(labs[i],3)))
labs[i] <- expression(paste0(mu,substring(labs[i],3)))
}
print(labs[i])
}
labs
}
data$dv <- factor(data$dv, labels = c("No", "Yes"))
fit <- rpart(dv ~ n + alpha + dev + mu, method="class", data=data)
rpart.plot(fit, yesno=2, box.palette = 0, extra=100, under = TRUE, split.fun = split.fun)
Neither the "expression" approach or "bquote" approach work. However, the split.fun function works fine as long as I just replace substrings with other strings (not expressions).
In trying to figure out what's going on, I've also been printing out the resulting labels. This is what I get:
[1] "root"
[1] "dev >= 0.075"
expression(paste0(mu, substring(labs[i], 3)))
expression(paste0(mu, substring(labs[i], 3)))
expression("alpha < 0.025")
expression("alpha >= 0.025")
expression("dev < 0.075")
expression("alpha < 0.025")
expression("dev >= 0.025")
expression(paste0(mu, substring(labs[i], 3)))
expression(paste0(mu, substring(labs[i], 3)))
expression("dev < 0.025")
expression("alpha >= 0.025")
From this, it seems that once I replace one label with an expression, all other labels are replaced with an expression.
Is there another approach to placing greek letters on the rpart.plot? Or is rpart.plot (or prp in general), simply not capable of including math expressions?

Answer 1

A combination fo #G5W's suggestion and fonts work. For those trying to do this, add the following to the top of the file:
library(extrafont)
loadfonts()
Then in adjust the rpart.plot call to use "Arial Unicode MS". This font seems to always correctly display math unicode characters (including combining characters).
rpart.plot(fit, yesno=2, box.palette = 0, extra=100, under = TRUE, split.fun = split.fun, split.font=1, split.family="Arial Unicode MS", family="Arial Unicode MS")