Special characters in ggplot labels

This is the code I used to create one of the figures in the manuscript “Accounting for drop-out using inverse probability censoring weights in longitudinal clustered data with informative cluster size”.

## here() starts at C:/Users/ayami/Desktop/Version Control/rcode
library(ggplot2)

themin <- min(allscens$value)
themax <- max(allscens$value)

# create labels for the factor variable
allscens$varlab <- factor(allscens$var, levels = c("alpha30", "alpha3-1", "alpha3-2"),
                          ordered = TRUE, labels=c("alpha[3] == 0", "alpha[3] == -1", "alpha[3] == -2"))

allscens$xnum <- factor(allscens$truth, levels = c("-2","-1.5","-1","-0.5","0"))

p <- ggplot(allscens, aes(x=xnum, y=value, fill=variable)) + 
  geom_boxplot() + 
  geom_hline(yintercept = 0, linetype = "longdash") +
  # bquote can also be used to mix special characters and text
  labs(y = "Relative bias", x =  bquote(alpha[1]~values)) + 
  theme_bw() +
  facet_wrap(~ varlab, scales = "free", labeller = label_parsed) +
  scale_fill_grey(start = 1, end = 0.4, 
                  name="Method",
                  breaks=c("ccbeta3relbias", "cwbeta3relbias", "bbdcwbeta3relbias", "rrzcwbeta3relbias"),
                  labels=c("CCGEE", "CWGEE", "DWGEE", "IPC-CWGEE")) +
  scale_y_continuous(limits = c(themin, themax)) 
p