[R] Special character is graph label

[Robert U]

Dear R users,
I am having issues finding a special character (and how to insert it) in the lab of a graph axis. 

Let us say that the label of my axis is "X", i would like the X to have a "line" over it, indicating that it is the "mean of X values" (i don't even know how to properly state that in english...). Does someone understand, and have any idea about how to do that?
Greetings,
R.H

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