Adding equations with Greek letters using geom_text() in ggplot

Question

I have a small data frame which I plot and to which I want to add an equation using greek letters

library(tidyverse)
Beta  <- c( 1.53,  1.36,  1.24, 1.17,  1.06, 0.92, 0.84, 0.76, 0.63, 0.48)
Sigma <- c( 0.49,  0.43,  0.39, 0.37,  0.33, 0.29, 0.27, 0.24, 0.20, 0.17)
df    <- data.frame(Beta, Sigma)

I then plot Sigma against Beta using the following code that I modeled on Adding equations to ggplots in R:

ggplot(df, aes(x = Beta, y = Sigma)) +
  geom_point() + 
  geom_smooth(method='lm', formula = y ~ x, se = FALSE, size = 0.6, color = "gray20") +
  ylab("Standard Deviation") +  
  geom_text(aes(1.0, 0.2, label=(paste(expression("sigma = 0.33 beta "*"")))),parse = TRUE) 

This works, and gives me the text string "sigma = 0.33 beta" on my plot, but what I really want is the greek letters mu and sigma, and not the words mu and sigma. Changing the expression to "$\sigma = 0.33 \beta "*""$ (or variants of this) only gives me an error: Error: '\s' is an unrecognized escape in character string starting ""$\s" What is the right syntax for adding greek letters (or symbols in general) to a plot using geom_text()? I've tried ?plotmath and it seems to suggest that alpha and beta will display as greek symbols, but they clearly do not.

Thanks in advance

Thomas Philips

Answer 1

When the plot coordinates or any other aesthetic element do not depend on the data, there is no need for aes(). This is the case for geom_text below, its arguments are all constants. And to parse the expression to be plotted as a text label is also not needed.

ggplot(df, aes(x = Beta, y = Sigma)) +
  geom_point() + 
  geom_smooth(method='lm', formula = y ~ x, se = FALSE, size = 0.6, color = "gray20") +
  ylab("Standard Deviation") +  
  geom_text(x = 1.0, y = 0.2, 
            label = expression(sigma == "0.33" ~ beta^"*"))