R: Programmatically changing ggplot scale labels to Greek letters with expressions - r

Question

I am trying to change the labels in a ggplot object to Greek symbols for an arbitrary number of labels. Thanks to this post, I can do this manually when I know the number of labels in advance and the number is not too large:
# Simulate data
df <- data.frame(name = rep(c("alpha1","alpha2"), 50),
value = rnorm(100))
# Create a plot with greek letters for labels
ggplot(df, aes(x = value, y = name)) + geom_density() +
scale_y_discrete(labels = c("alpha1" = expression(alpha[1]),
"alpha2" = expression(alpha[2])))
For our purposes, assume I need to change k default labels, where each of the k labels is the pre-fix "alpha" followed by a number 1:k. Their corresponding updated labels would substitute the greek letter for "alpha" and use a subscript. An example of this is below:
# default labels
paste0("alpha", 1:k)
# desired labels
for (i in 1:k) { expression(alpha[i]) }
I was able to hack together the below programmatic solution that appears to produce the desired result thanks to this post:
ggplot(df, aes(x = value, y = name)) + geom_density() +
scale_y_discrete(labels = parse(text = paste("alpha[", 1:length(unique(df)), "]")))
However, I do not understand this code and am seeking clarification about:
What is parse() doing here that expression() otherwise would do?
While I understand everything to the right-hand side of =, what is text doing on the left-hand side of the =?

Answer 1

Another option to achieve your desired result would be to add a new column to your data which contains the ?plotmath expression as a string and map this new column on y. Afterwards you could use scales::label_parse() to parse the expressions:
set.seed(123)
df <- data.frame(name = rep(c("alpha1","alpha2"), 50),
value = rnorm(100))
df$label <- gsub("^(.*?)(\\d+)$", "\\1[\\2]", df$name)
library(ggplot2)
library(scales)
ggplot(df, aes(x = value, y = label)) + geom_density() +
scale_y_discrete(labels = scales::label_parse())