Subscripts when using bquote in geom_text followed by text - ggplot2

Question

I've been looking through stackoverflow for the past few hours and couldn't find a solution which works for my problem.
The issue: I'm trying to use geom_text to manually write in a manually calculated value, which is fine when I just write the value and its unit using bquote (for example just writing 4.4µm is fine). However, I would like to write out D[g]= 4.4µm (with the g subsetted!) in the top left corner using geom_text. But once I start trying to add a subscript to it, it doesn't work and I get one of the following errors:
Error: Don't know how to add RHS to theme object
Error: Aesthetics must be either length 1 or the same as the data (14): label
Error: unexpected string constant in:
" #geom_text(label = bquote("D" [g] "= 4.4µm"), x = -.3, y = 900, size = 10) +
geom_text(label = bquote(D[g]"=4.4µm"
Here is the code I've been using thus far:
A = structure(list(`Size (µm)` = c(0.85, 0.85, 1.6, 1.6, 2.7, 2.7,
4, 4, 5.85, 5.85, 9, 9, 20, 20), `C/dlogd` = c(0, 70.7482842209313,
70.7482842209313, 147.721992341133, 147.721992341133, 752.133343128365,
752.133343128365, 296.076678012312, 296.076678012312, 226.648066580862,
226.648066580862, 302.286593848111, 302.286593848111, 0), `+s` = c(0,
175.47011068069, 175.47011068069, 243.15534458114, 243.15534458114,
1007.91042406178, 1007.91042406178, 439.475898343651, 439.475898343651,
366.578774657598, 366.578774657598, 385.065980566499, 385.065980566499,
0), `-s` = c(0, -33.9735422388272, -33.9735422388272, 52.2886401011273,
52.2886401011273, 496.356262194949, 496.356262194949, 152.677457680973,
152.677457680973, 86.7173585041254, 86.7173585041254, 219.507207129723,
219.507207129723, 0), Sampling = c("A", "A", "A", "A", "A", "A",
"A", "A", "A", "A", "A", "A", "A", "A"), Date = structure(c(1L,
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L), .Label = c("21st of February 2019",
"19th of June 2019", "3rd of July 2019", "17th of July 2019"), class = "factor"),
Datenoyear = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 1L), .Label = c("21st of February", "19th of June",
"3rd of July", "17th of July"), class = "factor"), DateNumber = c(21.2,
21.2, 21.2, 21.2, 21.2, 21.2, 21.2, 21.2, 21.2, 21.2, 21.2,
21.2, 21.2, 21.2)), row.names = c(NA, -14L), class = c("tbl_df",
"tbl", "data.frame"))
ggplot(A, aes(x= `Size (µm)`)) +
geom_line(aes(y = `C/dlogd`), size = 1.5) +
geom_line(aes(y = `+s`), linetype = "twodash", color = "red") +
geom_line(aes(y = `-s`), linetype = "dashed" , color = "blue") +
geom_point(aes(y = `+s`), color = "red") +
geom_point(aes(y = `C/dlogd`)) +
geom_point(aes(y = `-s`), color = "blue") +
scale_x_log10(limits = c(0.4,21)) +
#geom_text(label = bquote("D" [g] "= 4.4µm"), x = -.3, y = 900, size = 10) + #gives the second error
#geom_text(label = bquote(D[g]"=4.4µm"), x = -.1, y = 900, size = 10) + #gives the 3rd error
# geom_text(label = bquote("4.4µm"), x = -0.3, y = 900, size = 10) + #No error, but doesn't give me the "D[g] = " I need
theme_bw() +
annotation_logticks(side = "b") +
facet_grid(~Datenoyear) +
xlab(element_blank()) +
ylab(element_blank())
What is it that I'm doing wrong? I've tried looking at these solutions:
http://rchaeology.blogspot.com/2012/11/combining-superscripts-subscripts-and.html
https://community.rstudio.com/t/use-bquote-in-ggplot2-for-subscript-text/40882
writing a label in R for a plot using text, and subscripts using either bquote, paste or expression
But unfortunately none of them have worked. Any help would be appreciated.

Answer 1

Unfortunately I can't tell you what the problem is with your code. But instead of using bquote the desired result can be achieved using as.character(expression(paste(D[g], "=4.4µm"))). Source: see here. Try this:
ggplot(A, aes(x= `Size (µm)`)) +
geom_line(aes(y = `C/dlogd`), size = 1.5) +
geom_line(aes(y = `+s`), linetype = "twodash", color = "red") +
geom_line(aes(y = `-s`), linetype = "dashed" , color = "blue") +
geom_point(aes(y = `+s`), color = "red") +
geom_point(aes(y = `C/dlogd`)) +
geom_point(aes(y = `-s`), color = "blue") +
geom_text(label = as.character(expression(paste(D[g], "=4.4µm"))), parse = TRUE, x = -.1, y = 900, size = 10) + #gives the 3rd error
scale_x_log10(limits = c(0.4, 21)) +
theme_bw() +
annotation_logticks(side = "b") +
facet_grid(~Datenoyear) +
xlab(element_blank()) +
ylab(element_blank())