ggplot right align axis text when using expression - r

Question

I am making a bar chart with long axis labels which i need to wrap and right align. The only complication is i need to add a expression to have superscripts.
library(ggplot2)
library(scales)
df <- data.frame("levs" = c("a long label i want to wrap",
"another also long label"),
"vals" = c(1,2))
p <- ggplot(df, aes(x = levs, y = vals)) +
geom_bar(stat = "identity") +
coord_flip() +
scale_x_discrete(labels = wrap_format(20))
which produces the desired result:
with properly wrapped text with all labels fully right aligned.
However now I attempt to add superscript using the below code, and the axis text alignment changes:
p <- ggplot(df, aes(x = levs, y = vals)) +
geom_bar(stat = "identity") +
coord_flip() +
scale_x_discrete(labels = c(expression("exponent"^1),
wrap_format(20)("another also long label")))
(NB I cannot use unicode as is recommended to others with the same question because it does not work with the font I am required to use).
How can I get the axis text to be aligned right even when one of the axis labels includes an expression?

Answer 1

It's a strange thing, but if a vector (e.g. a character vector of labels) includes an object created by expression(), the whole vector appears to be treated as an expression:
# create a simple vector with one expression & one character string
label.vector <- c(expression("exponent"^1),
wrap_format(20)("another also long label"))
> sapply(label.vector, class) # the items have different classes when considered separately
[1] "call" "character"
> class(label.vector) # but together, it's considered an expression
[1] "expression"
... and expressions are always left-aligned. This isn't a ggplot-specific phenomenon; we can observe it in the base plotting functions as well:
# even with default hjust = 0.5 / vjust = 0.5 (i.e. central alignment), an expression is
# anchored based on the midpoint of its last line, & left-aligned within its text block
ggplot() +
annotate("point", x = 1:2, y = 1) +
annotate("text", x = 1, y = 1,
label = expression("long string\nwith single line break"))+
annotate("text", x = 2, y = 1,
label = expression("long string\nwith multiple line\nbreaks here")) +
xlim(c(0.5, 2.5))
# same phenomenon observed in base plot
par(mfrow = c(1, 3))
plot(0, xlab=expression("short string"))
plot(0, xlab=expression("long string\nwith single line break"))
plot(0, xlab=expression("long string\nwith multiple line\nbreaks here"))
Workaround
If we can force each label to be considered on its own, without the effect of other labels in the label vector, the non-expression labels could be aligned like normal character strings. One way to do this is to convert the ggplot object into grob, & replace the single textGrob for y-axis labels with multiple text grobs, one for each label.
Prep work:
# generate plot (leave the labels as default)
p <- ggplot(df, aes(x = levs, y = vals)) +
geom_bar(stat = "identity") +
coord_flip()
p
# define a list (don't use `c(...)` here) of desired y-axis labels, starting with the
# bottom-most label in your plot & work up from there
desired.labels <- list(expression("exponent"^1),
wrap_format(20)("another also long label"))
Grob hacking:
library(grid)
library(magrittr)
# convert to grob object
gp <- ggplotGrob(p)
# locate label grob in the left side y-axis
old.label <- gp$grobs[[grep("axis-l", gp$layout$name)]]$children[["axis"]]$grobs[[1]]$children[[1]]
# define each label as its own text grob, replacing the values with those from
# our list of desired y-axis labels
new.label <- lapply(seq_along(old.label$label),
function(i) textGrob(label = desired.labels[[i]],
x = old.label$x[i], y = old.label$y[i],
just = old.label$just, hjust = old.label$hjust,
vjust = old.label$vjust, rot = old.label$rot,
check.overlap = old.label$check.overlap,
gp = old.label$gp))
# remove the old label
gp$grobs[[grep("axis-l", gp$layout$name)]]$children[["axis"]]$grobs[[1]] %<>%
removeGrob(.$children[[1]]$name)
# add new labels
for(i in seq_along(new.label)) {
gp$grobs[[grep("axis-l", gp$layout$name)]]$children[["axis"]]$grobs[[1]] %<>%
addGrob(new.label[[i]])
}
# check result
grid.draw(gp)