display double dot over character in plotmath?

On May 13, 2017, at 5:47 AM, Ranjan Maitra <maitra at email.com> wrote:

On Fri, 12 May 2017 23:39:14 -0700 Daniel Nordlund <djnordlund at gmail.com> wrote:

On 5/12/17 4:55 PM, Ranjan Maitra wrote:

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~"?"))))

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

You haven't told what OS you are using, but with Windows OS, you can get 
the '?' by making sure the NUMLOCK key is on, hold down the alt key and 
press 0228 on the numeric keypad.

I am sorry, I use a linux operating system. I use Fedora 25 but the student I wanted to show this uses Ubuntu, though I don't know if the distribution matters.

Thanks again for your help, and best wishes,
Ranjan