strip.text does not work with labeller - facet_wrap ggplot2

Question

I am using facet_wrap and need expression in header so I am using labeller. But then strip.text does not work (I want the header to be in bold). see code:

sc <- c("Ref","DJ","DT","DTHW")
vars <- c("O2","PO4","NH4","NO3")
df1 <- data.frame(scenario=rep(sc,4),variable=rep(vars,each=4),pct=runif(16,-10,10))

ggplot(df1, aes(y= scenario, x=pct)) +
  geom_col(aes( fill = scenario), position = position_dodge()) + 
  facet_wrap(vars(variable)) +
  theme (strip.text = element_text(face = "bold"))

This gives bold header as expected:

But if I add the expression with labeller:

vars2plot <- c(expression(paste("Hypol. ",O[2])),expression(paste("Epil. ",PO[4]^"3-")),
               expression(paste("Epil. ",NH[4]^"+")), expression(paste("Hypol. ",NO[3]^"-")))
names(vars2plot) <- vars
df1$variable <- factor(df1$variable, levels = vars, labels = vars2plot)

ggplot(df1, aes(y= scenario, x=pct)) +
  geom_col(aes( fill = scenario), position = position_dodge()) + 
  facet_wrap(vars(variable), labeller = labeller(variable=label_parsed)) +
  theme (strip.text = element_text(face = "bold"))

The bold does not appear:

Any ideas how to make the header bold?

Answer 1

If you use expressions, then you need to use the plotmath syntax to make things bold. You cannot use font properties to alter the appearance. You can wrap your expressions in bold() with

vars2plot <- c(expression(paste("Hypol. ",O[2])),expression(paste("Epil. ",PO[4]^"3-")),
               expression(paste("Epil. ",NH[4]^"+")), expression(paste("Hypol. ",NO[3]^"-")))

# or simplified ...
# vars2plot <- expression(
#   "Hypol. " ~ O[2],
#   "Epil. " ~ PO[4]^"3-",
#   "Epil. " ~ NH[4]^"+",
#   "Hypol. " ~ NO[3]^"-")

# now make them all bold
vars2plot <- as.expression(lapply(vars2plot, function(x) bquote(bold(.(x)))))

which will output