Combining new lines and italics in facet labels with ggplot2

Question

I have a problem getting some words used in facet labels in italics. I use the following code to create new lines for the labels:

    levels(length_subject$CONSTRUCTION) <- 
c("THAT \n Extraposed", "THAT \n Post-predicate", "TO \n Extraposed \n for-subject", "TO \n Post-predicate \n for-subject", "THAT \n Extraposed \n that-omission", "THAT \n Post-predicate \n that-omission")

However, I want the words "that" and "for" to appear in italics. I've tried something like

"TO \n Extraposed \n (italics(for))-subject"

bit it doesn't work.

This is what the plots look like:

produced with the following code:

ggplot( length_subject, aes( x = SUBJECT ) ) +
  geom_histogram(binwidth=.6, colour="black", fill="grey") +
  ylab("Frequency") +  
  xlab("Subject length") +  
  scale_x_discrete(breaks=c(2,4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30)) + #
  facet_grid( SUBJECT_TYPE~CONSTRUCTION, scales="free_x", space="free") +
  theme(strip.text.x = element_text(size = 8)) 

Here is a reduced variant of the data:

structure(list(ID = structure(1:86, .Label = c("A05_122_01", 
"A05_253_01", "A05_277_07", "A05_400_01", "A05_99_01", "A06_1076_01", 
"A06_1261_01", "A06_1283_01", "A06_1283_02", "A06_1317_01", "A06_1326_01", 
"A06_1389_01", "A06_1390_01", "A06_1437_01", "A06_1441_02", "A06_1441_03", 
"A06_1442_03", "A06_1456_01", "A06_1461_01", "A06_830_01", "A06_868_01", 
"A06_884_01", "A06_884_03", "A0K_1057_02", "A0K_1144_07", "A0K_1177_01", 
"A0K_1190_03", "A0K_1214_03", "A0K_1216_01", "A0K_950_02", "A0K_986_01", 
"A1A_102_02", "A1A_163_01", "A1A_199_01", "A1A_45_01", "A1A_97_01", 
"A1B_1008_02", "A1B_1013_01", "A1B_1028_02", "A1B_1042_01", "A1B_1064_01", 
"A1B_1126_03", "A1B_1152_01", "A1B_1174_01", "A1B_1271_01", "A1B_997_01", 
"A1J_487_01", "A1J_544_02", "A1J_555_03", "A1J_569_01", "A1J_601_01", 
"A1N_422_04", "A1N_70_02", "A1S_191_01", "A1S_329_01", "A1S_330_01", 
"A1S_465_04", "A1Y_248_01", "A1Y_278_02", "A1Y_292_01", "A1Y_466_01", 
"A1Y_521_01", "A1Y_612_01", "A1Y_634_01", "A26_139_03", "A26_142_01", 
"A26_148_01", "A26_289_01", "A26_345_02", "A26_439_01", "A26_441_02", 
"A26_463_01", "A28_171_01", "A28_244_01", "A28_245_01", "A28_30_01", 
"A28_341_01", "A28_42_01", "A28_494_03", "A2A_301_01", "A2A_396_01", 
"A2A_599_01", "A2A_637_01", "A2A_676_01", "A2E_22_01", "A2E_25_03"
), class = "factor"), SUBJECT = c(3L, 2L, 6L, 2L, 2L, 1L, 1L, 
1L, 1L, 2L, 4L, 1L, 4L, 2L, 3L, 2L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 
1L, 7L, 1L, 3L, 2L, 2L, 1L, 6L, 7L, 4L, 1L, 5L, 4L, 2L, 9L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 3L, 5L, 3L, 4L, 1L, 1L, 1L, 1L, 5L, 
2L, 1L, 1L, 2L, 1L, 1L, 1L, 1L, 5L, 2L, 1L, 2L, 2L, 1L, 7L, 1L, 
4L, 1L, 2L, 1L, 1L, 3L, 1L, 13L, 2L, 1L, 1L, 1L, 3L, 1L, 1L), 
    CONSTRUCTION = structure(c(1L, 3L, 1L, 1L, 1L, 4L, 4L, 1L, 
    1L, 5L, 5L, 1L, 1L, 5L, 1L, 3L, 5L, 1L, 5L, 4L, 3L, 3L, 1L, 
    5L, 3L, 5L, 1L, 1L, 2L, 3L, 1L, 1L, 3L, 1L, 1L, 1L, 3L, 1L, 
    4L, 3L, 1L, 3L, 1L, 1L, 1L, 1L, 4L, 2L, 4L, 1L, 1L, 3L, 2L, 
    5L, 1L, 1L, 1L, 3L, 1L, 1L, 4L, 4L, 3L, 1L, 2L, 3L, 3L, 1L, 
    3L, 1L, 1L, 1L, 6L, 1L, 1L, 2L, 4L, 4L, 3L, 5L, 3L, 3L, 3L, 
    3L, 5L, 1L), .Label = c("THAT_EXT", "THAT_EXT_NT", "THAT_POST", 
    "THAT_POST_NT", "TO_EXT_FOR", "TO_POST_FOR"), class = "factor"), 
    SUBJECT_TYPE = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 
    2L, 1L, 1L, 2L, 1L, 1L, 1L, 1L, 1L, 2L, 1L, 2L, 2L, 2L, 2L, 
    1L, 1L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 1L, 3L, 1L, 1L, 
    2L, 3L, 1L, 2L, 2L, 3L, 1L, 3L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 
    1L, 1L, 1L, 2L, 2L, 3L, 2L, 2L, 2L, 3L, 1L, 1L, 2L, 1L, 1L, 
    2L, 1L, 2L, 1L, 2L, 1L, 2L, 2L, 1L, 2L, 1L, 1L, 2L, 2L, 2L, 
    1L, 3L, 3L), .Label = c("NP", "PRO", "PROPER"), class = "factor")), .Names = c("ID", 
"SUBJECT", "CONSTRUCTION", "SUBJECT_TYPE"), class = "data.frame", row.names = c(NA, 
-86L))

Answer 1

To get italics, you need the formatting described in plotmath (and then for that to be parsed as an expression). However, the plotmath syntax does not have a line break operation. You can get something similar with atop, though. With your given example, you can set the labels to

levels(length_subject$CONSTRUCTION) <- 
  c("atop(textstyle('THAT'),textstyle('Extraposed'))", 
    "atop(textstyle('THAT'),textstyle('Post-predicate'))",
    "atop(atop(textstyle('TO'),textstyle('Extraposed')),italic('for')*textstyle('-subject'))",
    "atop(atop(textstyle('TO'),textstyle('Post-predicate')),italic('for')*textstyle('-subject'))",
    "atop(atop(textstyle('THAT'),textstyle('Extraposed')),italic('that')*textstyle('-omission'))",
    "atop(atop(textstyle('THAT'),textstyle('Post-predicate')),italic('that')*textstyle('-omission'))")

and then adding labeller=label_parsed to the facet_grid call

ggplot( length_subject, aes( x = SUBJECT ) ) +
  geom_histogram(binwidth=.6, colour="black", fill="grey") +
  ylab("Frequency") +  
  xlab("Subject length") +  
  scale_x_discrete(breaks=c(2,4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30)) + #
  facet_grid( SUBJECT_TYPE~CONSTRUCTION, scales="free_x", space="free", 
              labeller=label_parsed) +
  theme(strip.text.x = element_text(size = 8)) 

gives

It's not perfect (the spacing between lines is not the same, and the disparity would only get worse the more lines there are), but that is the only way I've found to combine the two (newlines in plotmath expressions).