R ggplot2 using variable expressions in legend name

Question

I am trying to plot a series of time series with ggplot2 that sometimes have greek names. As the plot uses dynamic names (i.e. the names of the variables are stored within another variable) I am having troubles to get it to work.

Here is an example:

# create some data
df <- data.frame(time = rep(1:10, 3), 
                 variable = rep(letters[1:3], each = 10), 
                 val = rnorm(30))

# create a variable for the group name
nam <- expression("alpha[i]")

library(ggplot2)
# plot the data as a line
ggplot(df, aes(x = time, y = val, color = variable)) + 
   geom_line() + 
   # Option 1: Does not work
   scale_color_discrete(name = eval(nam)) 
   # Option 2: works but has no variable input
   # scale_color_discrete(name = expression(alpha[i])) 

Do you have any idea of how I can evaluate the variable nam to be displayed as the name of the legend as in option 2? Thank you very much!

Answer 1

Using this code

# create some data
df <- data.frame(time = rep(1:10, 3), 
                 variable = rep(letters[1:3], each = 10), 
                 val = rnorm(30))

# create a variable for the group name
nam <- expression(alpha[i])

library(ggplot2)
# plot the data as a line
ggplot(df, aes(x = time, y = val, color = variable)) + 
  geom_line() + 
  # Option 1: Does not work
  scale_color_discrete(name = nam) 
# Option 2: works but has no variable input
#  scale_color_discrete(name = expression(alpha[i]))

This gives me the plot you probably wanna see. No eval in name = eval(name) and no blockquotes in the assignment nam <- expression(alpha[i])