Pass an expression in geom_text using both a variable and greek letter

May 31, 2022

I am trying to add text to a plot which includes both a variable and a greek letter. I would like to display the values that the variables pt and P_T take on but I’d like the greek letter to be used for alpha. The code below gives the following error:

Error in f(): ! Aesthetics must be valid data columns. Problematic
aesthetic(s): label = expression(…). Did you mistype the name of a
data column or forget to add after_stat()?

Any help would be greatly appreciated!

x <- rep(seq(1,5),3)
value <- runif(15)
alpha <- rep(c(0.5,0.6,0.7),each=5)
my_data <- data.frame(x=x,value=value,alpha=alpha)
pt=6
P_T=0.7


ggplot(data=my_data, aes(x=x, y=value,group=as.factor(alpha),color=as.factor(alpha)))+
  geom_line()+
  scale_x_continuous(name =paste("True Landmark PFS at", pt, "Months"), breaks=seq(1,5,1))+
  theme(plot.margin= unit(c(0.7, 0.7, 0.7,0.7), "cm"))+
  theme(plot.title = element_text(hjust = 0.5, size=12))+
  scale_y_continuous(name ="Probability of ESOE",breaks=seq(0,1,by=0.25),limits=c(0,1))+
  guides(color = guide_legend(title = expression(alpha)))
  geom_text(aes(x=3, label=expression(paste("Posterior prob (True RMST at",pt,"month>",alpha,"|data>",P_T)), y=0.75),
            angle=0, size=2.5, color="black")

>Solution :

If you just want to display a greek letter the one option would be to simply use the unicode, e.g. \u03B1 for alpha. Also, as you want to add just one annotation to your plot I would suggest to use annotate as with geom_text you add the annotation multiple times:

set.seed(123)

library(ggplot2)

ggplot(data = my_data, aes(x = x, y = value, group = as.factor(alpha), color = as.factor(alpha))) +
  geom_line() +
  scale_x_continuous(name = paste("True Landmark PFS at", pt, "Months"), breaks = seq(1, 5, 1)) +
  theme(plot.margin = unit(c(0.7, 0.7, 0.7, 0.7), "cm")) +
  theme(plot.title = element_text(hjust = 0.5, size = 12)) +
  scale_y_continuous(name = "Probability of ESOE", breaks = seq(0, 1, by = 0.25), limits = c(0, 1)) +
  guides(color = guide_legend(title = expression(alpha))) +
  annotate(geom = "text", x = 3, label = paste("Posterior prob (True RMST at", pt, "month > \u03B1 | data>", P_T), y = 0.75,
    angle = 0, size = 2.5, color = "black"
  )