X̅ and R chart

I'm not sure about the best way to display multi-line annotations. If you stick to a single line you can get a pretty readable annotation using:

xs = ys = 1:10 
x = 3.2
y = 4.6
plot(xs, ys)
text(x, y - 0.2, 
     bquote("(" * bar(x) * " = " * .(x) * ", " *  bar(y) * " = " * .(y) * ")")
)

Producing:

Use \bar for short lines and \overline for longer lines.

A separate comment: In fine math typography, a \cdot is generally preferred over * to denote multiplication.

\documentclass[11pt]{report}
\usepackage[utf8]{inputenc}
\usepackage{mathtools,array}
\begin{document}

\begin{align*}
C_f 
&= 100 - 100\cdot\frac{\overline{R_{j(pr)}}}{\overline{R_{j(obs)}}} \\
&= 100 - 100\cdot\frac{\bar{R}_{j(pr)}}{\bar{R}_{j(obs)}}
\end{align*}
\end{document}

The bar notation is often used in Lie Group theory.

For example, the $n \times n$ unitary matrices are defined as

$$U(n) := \{ X \in \mathrm{Mat}(n,\mathbb C): \overline{X}^{\top}\!X = E\}$$

where $\overline{X}^{\top}$ is the conjugate matrix of $X$ transposed, and $E$ is the identity matrix.

Just as the (real) orthogonal matrices $X^{\top}\!X=E$ preserve the (real) inner product $$\langle u,v\rangle = u^{\top}v$$ the (complex) unitary matrices $\overline{X}^{\top}\!X = E$ preserve the (complex) Hermitian product $$\langle u,v\rangle = \overline{u}^{\top}v$$

Could you tell us what $\mathbb{E}$ is, e.g. is it the identity matrix? If it is then I suspect that $\overline{X}$ might be a, potentially confusing, alternative to $\overline{X}^{\top}$. I've seen $\overline{X}^{\top}$ written as $X^*$ or $X^{\dagger}$ but never just $\overline{X}$.

Here is a command \xoverline[width percent]{symb} that will do it. Note that it will not scale inside sub or superscripts. If you need that, everthing has to go through a \mathchoice resulting in a lot more complex code.

\documentclass{article}
\usepackage{amsmath,amssymb}
\makeatletter
\newsavebox\myboxA
\newsavebox\myboxB
\newlength\mylenA

\newcommand*\xoverline[2][0.75]{%
    \sbox{\myboxA}{$\m@th#2$}%
    \setbox\myboxB\null% Phantom box
    \ht\myboxB=\ht\myboxA%
    \dp\myboxB=\dp\myboxA%
    \wd\myboxB=#1\wd\myboxA% Scale phantom
    \sbox\myboxB{$\m@th\overline{\copy\myboxB}$}%  Overlined phantom
    \setlength\mylenA{\the\wd\myboxA}%   calc width diff
    \addtolength\mylenA{-\the\wd\myboxB}%
    \ifdim\wd\myboxB<\wd\myboxA%
       \rlap{\hskip 0.5\mylenA\usebox\myboxB}{\usebox\myboxA}%
    \else
        \hskip -0.5\mylenA\rlap{\usebox\myboxA}{\hskip 0.5\mylenA\usebox\myboxB}%
    \fi}
\makeatother
\begin{document}

$|\xoverline{W}|~~
 |\xoverline{i}|~~
 |\xoverline[3.0]{i}|$

\bigskip

$\bar{\mathbb{R}}~~\overline{\mathbb{R}}~~\xoverline{\mathbb{R}}$

\end{document}