Unicode characters in ggplot legend

问题:

I am trying to plot some graph using ggplot2 in R and label the legends using unicode characters such as Japanese. The following is my code:

ggplot(mtcars, aes(x=mpg, y=wt, colour = 'ああ')) + geom_line()

But I end up getting the following plot:

The legend is not displayed properly. Could anyone tell me how to fix this? Thank you so much!!