Parsing a ggplot2 title containing subscripts with comma directly following number

I am looking to add subscripts to a chart title in ggplot, to look something like:

The values of k₁, k₂ and k₃ are stored in variables: k1, k2 and k3

Reproducible example

data <- data.frame(age = 30:32, count = 2:4)

k1 <- 2
k2 <- 9 
k3 <- 4

Possible method: Parsing text

I have been able to add in the variable names and values by pasting together an expression and parsing it, as explained here:

title <- paste0("k[1]*", k1, "~k[2]*", k2, "~k[3]*", k3)
title_parsed <- parse(text = title)

data %>% 
   ggplot(aes(x = age, y = count)) + 
   geom_point() +
   ggtitle(title_parsed)

which gives:

Another possible method: Using bquote

And have been able to get the fixed parts of the title (equals signs and comma) to display correctly using bquote (but cannot add in the values of k from my variables):

data %>% 
   ggplot(aes(x = age, y = count)) + 
   geom_point() +
   ggtitle(bquote(k[1]*" = ,"  ~k[2]*" = and" ~ k[3]*" = "))

I have also been able to combine these to produce a short title, with a single k value:

title <- paste0("k[1]*", "\" = \"*", k1)
title_parsed <- parse(text = title)

data %>% 
   ggplot(aes(x = age, y = count)) + 
   geom_point() +
   ggtitle(title_parsed)

But for longer titles with multiple k values, I have been unable to parse a text string with a number next to a comma. (e.g. both of the below values of title throw an error when fed into the parse function)

title <- paste0("k[1]*", "\" = \"*", k1, "*,")
title <- paste0("k[1]*", "\" = \"*", k1, ",")

title_parsed <- parse(text = title)

The issue seems unique to commas and other special characters, as I can add other text after the number:

title <- paste0("k[1]*", "\" = \"*", k1, "~text")
title_parsed <- parse(text = title)

What am I doing wrong? I wondered if I needed to use escape characters before the comma, but no combination of \, \\ and \\\ seems to work.

>Solution :

You can use the .() operator inside bquote to refer to an external variable. Additionally, you could use the == operator to generate an equals sign.

data %>% 
  ggplot(aes(x = age, y = count)) + 
  geom_point() +
  theme_gray(20) +
  ggtitle(bquote(k[1] == .(k1) * "," ~ k[2] == .(k2) * " and" ~ k[3] == .(k3)))