r - ggplot scale_x_continuous with symbol: make bold

Question

Here is the issue: I am using ggplot to to make a graph in which the x axis ranges from -90 to 90 degrees. I have successfully used a function to add degree symbols to the tick labels created by scale_x_continuous. However, when I adjust the font face using theme(), the tick labels do not become bold. Changing the font size works, however. Here is a reproducible example:

# function to add degree symbols
add_degree <- function(x) {
  parse(text = paste(x, "*degree",sep = ""))
}

y<- c(0.00552243346007605, 0.00553155893536122, 0.005693536121673, 
        0.0054722433460076, 0.00562661596958175, 0.00546768060836502, 
        0.00561292775665399, 0.00550950570342205, 0.0056851711026616, 
        0.00551558935361217, 0.0055041825095057, 0.00556501901140684, 
        0.00552699619771863, 0.00552623574144487, 0.0055680608365019, 
        0.00567148288973384, 0.00550342205323194, 0.00553764258555133, 
        0.00538098859315589, 0.00573307984790875, 0.00564486692015209, 
        0.00561444866920152, 0.00556349809885932, 0.00571254752851711, 
        0.00544030418250951, 0.00557946768060837, 0.00546083650190114, 
        0.00549049429657795, 0.00557414448669202, 0.00553916349809886, 
        0.00551787072243346, 0.00557794676806084, 0.0055041825095057, 
        0.00552699619771863, 0.0056509505703422, 0.00544790874524715, 
        0.00555555555555556)

x <- seq(-90,90,by = 5)
dat <- data.frame(x,y)
ggplot(dat,aes(x = x, y = y))+
  geom_line()+
  scale_x_continuous(limits = c(-90,90), breaks = seq(-90,90, by = 15), labels = add_degree)+
  theme_bw() +
  theme(

    # Adjust axis label and title font sizes below
    axis.title = element_text(size=12, face= "bold"),
    plot.title = element_text(size=12, face = "bold"),
    # Adjust size of tick labels
    axis.text.x = element_text(size=12, face = "bold"),
    axis.text.y = element_text(size=12, face = "bold"), 
    panel.grid.major = element_blank(),
    panel.grid.minor = element_blank(),
    panel.border = element_blank(),
    panel.background = element_blank(),
    axis.line = element_line(colour = "black")
  )

I have tried adjusting the parse function above, but no luck so far. I have looked at the following solution, but have not been able to get it to work in ggplot: How to make beta italic and bold in axis label and P italic and bold in text

Here is my output for sessionInfo():

R version 3.2.2 (2015-08-14)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 14.04.3 LTS

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8   
 [6] LC_MESSAGES=en_US.UTF-8    LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

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

other attached packages:
[1] reshape2_1.4.1 ggplot2_1.0.1  moments_0.14  

loaded via a namespace (and not attached):
 [1] Rcpp_0.12.0      digest_0.6.8     MASS_7.3-44      grid_3.2.2       plyr_1.8.3       gtable_0.1.2     magrittr_1.5     scales_0.2.5     stringi_0.5-5   
[10] proto_0.3-10     labeling_0.3     tools_3.2.2      stringr_1.0.0    munsell_0.4.2    colorspace_1.2-6

P.s. I believe this is a very similar (if not identical) question to the following unanswered question: making y-axis labels bold in ggplot (x-axis is set bold but y-axis label doesn't change).

Any help would be greatly appreciated!

Answer 1

I'm reusing the function taken from a similar question, which only requires a little adjustment.

make_labels <- function(value) {
  x <- as.character(value)
  do.call(expression, lapply(x, function(y) bquote(bold(.(y))^degree)))
}

Now, in your ggplot call instead of add_degree use labels = make_labels and you'll get