How to use subscript in titles of facet grid in ggplot2

Question

I am unable to create a subscript for the last subtitle (pm10). How can I capitalize pm and create subscript "10". Presently the title is pm10.

library(dlnm)
library(ggplot2)
library(reshape2)
library(plyr)
data <- chicagoNMMAPS
data1<-data[c(1,7:9,13)]
dmelt = melt(data1, id.vars = 'date')


ggplot(dmelt, aes(x = date, y = value)) + 
geom_line() + 
theme_bw() +
facet_wrap(~ variable, scales = 'free_y', ncol = 1)+ 
theme(strip.text.x = element_text(size=14,face="bold"),
strip.background = element_rect(colour="")) + 
labs(x = "Date") +
theme(axis.title=element_text(face="bold",size="14"),axis.text=element_text(size=14,face="bold")) +
theme(#panel.grid.major = element_blank(),
panel.grid.minor = element_blank(),
strip.background = element_blank(),
panel.border = element_rect(colour = "black"))

Answer 1

So, you can do this using a labeller function. Unfortunately, facet_wrap does not currently have this, but facet_grid does.

vnames <-list(
'death' = 'death',
'cvd' = 'cvd',
'resp' = 'resp',
'pm10' = bquote(pm[10]))

vlabeller <- function(variable,value){
  return(vnames[value])
}

bquote is a flexible way to use expressions. More can be found here. You can see more Then, simply change the original facet_wrap function to the following:

facet_grid(variable~., scales = 'free_y', labeller = vlabeller)

However, its appearance is a bit different than what you may want. In that case, you can deal directly with the elements of the plot itself, but it's much more cumbersome. The method is described here: https://stackoverflow.com/a/19298442/1362215