How to show the Mean (mu) and the Standard Deviation (Sigma) symbol in ggplot.

youtube video link with explanations for these examples https://youtu.be/gA0jYDP6XhA

#install.packages("ggplot2")
library(ggplot2)
library(stringr)

# Using the built in dataset chickwts

df <- chickwts

# Let us make the feed types to start with capital characters(Title case)

df$feed <- str_to_title(df$feed) 


# Calculate overall mean and standard deviation 

mean <- round( mean(df$weight),1)
sd   <- round( sd  (df$weight),1)


# Calculate the values which are one sd away from the mean
Mean_plus_SD  =   mean + sd
Mean_minus_SD =   mean - sd



#Plot the chart

pl <- ggplot(data= df, aes(x = feed, y =  weight))
pl <- pl + geom_point(aes(size = weight, colour = weight), alpha = 0.8,show.legend = FALSE)
pl <- pl + theme_classic()
pl <- pl + scale_color_gradient(low ="red",  high = "green")
pl <- pl + labs(title = "Weight distribution based on Chicken feeds")
pl

pl <- ggplot(data= df, aes(x = feed, y =  weight))
pl <- pl + geom_point(aes(size = weight, colour = weight), alpha = 0.8,show.legend = FALSE)
pl <- pl + geom_hline(yintercept = mean, colour = "blue",  linetype ="dashed")
pl <- pl + geom_label(x = 1.5, y = mean,  label = as.expression(bquote(mu == .(mean))),vjust = -0.5, color = "blue")
pl <- pl + theme_classic()
pl <- pl + scale_color_gradient(low ="red",  high = "green")
pl <- pl + labs(title = "Weight distribution based on Chicken feeds")
pl <- pl + labs(subtitle = "Scatter Plot showing the mean")
pl

pl <- ggplot(data= df, aes(x = feed, y =  weight))
pl <- pl + geom_point(aes(size = weight, colour = weight), alpha = 0.8,show.legend = FALSE)

pl <- pl + geom_hline(yintercept = mean, colour = "blue",  linetype ="dashed")
pl <- pl + geom_label(x = 1.5, y = mean,  label = as.expression(bquote(mu == .(mean))),vjust = -0.5, color = "blue")

pl <- pl + geom_hline(yintercept = Mean_plus_SD, colour = "red",  linetype ="dashed")
pl <- pl + geom_text(x = 1.5, y = Mean_plus_SD,  label = as.expression(bquote(mu + sigma == .(Mean_plus_SD))),vjust = -0.5, color = "red")

pl <- pl + geom_hline(yintercept = Mean_minus_SD, colour = "red",  linetype ="dashed")
pl <- pl + geom_text(x = 1.5, y = Mean_minus_SD,  label = as.expression(bquote(mu - sigma == .(Mean_minus_SD))),vjust = -0.5, color = "red")

pl <- pl + theme_classic()
pl <- pl + scale_color_gradient(low ="red",  high = "green")
pl <- pl + labs(title = "Weight distribution based on Chicken feeds")
pl <- pl + labs(subtitle = "Scatter Plot showing the mean and mean + 1 Std. Dev. and mean - 1 Std. Dev. lines")
pl