[R] display double dot over character in plotmath?

[Ranjan Maitra]

Hi,

Is it possible to display double dot (umlaut) over a character such as would be possible using \ddot x in LaTeX? I can do this using tikzDevice but I wanted something simpler to point to. 

Here is an example of what I would like to do, but it is not quite there:

require(ggplot2)
data<-as.data.frame(c("a","b","c","a","b","c"))
colnames(data)<-"Y"
data$X<-c(1:6)
data$Z<-c(1,2,3,1,2,3)

ggplot(data, aes(x=X)) + geom_line(aes(y = Z), size=0.43) + xlab(expression(atop(top,bold(Age~"à"))))

I would like to put in a double dot over the "a" in the x-axis instead of "`".

Many thanks for any suggestions and best wishes,
Ranjan

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