I'm not sure if it's the one you're looking for, but
process.capability() is a function in the
qcc package ( https://CRAN.R-project.org/package=qcc). According to the documentation for
process.capability() (see link above), it:
Computes process capability indices for a 'qcc' object of type
"xbar" and plot the histogram.
I think you might have missed it due to a spelling error.
There seem to be quite a few functions with some variation of the term "xbar" in them, so you might want to look around.
qcc was the only package I found with an actual
process.capability() function, though. In addition to its CRAN page, and GitHub repo (below), qcc also has a pkgdown site with a nice Getting started vignette.
Edit: Note, I edited the title of your question to use lowercase letters for
xbar. R is case sensitive, so if you were actually typing in
XBAR, it would not be able to find the