\documentclass[9pt,letterpaper]{amsart}
\usepackage[margin=1.2in]{geometry}
\usepackage{calc}

% Usage: \asdf{text}{lift fraction}{width fraction}{thickness}
\newcommand{\asdf}[4]{%
 \makebox[0pt][l]{%
   \makebox[\widthof{#1}][c]{
     \rule[\heightof{#1}*\real{#2}]{\widthof{#1}*\real{#3}}{#4}}%
 }#1}

\begin{document}
\noindent
Using $\backslash$bar:
$$\bar{X}\qquad \bar{M}\qquad \bar{x} \qquad \bar{y}$$
Using $\backslash$overline:
$$\overline{X}\qquad\overline{M}\qquad\overline{x}\qquad\overline{y}$$

Top gives width fraction

Side gives height fraction

Thickness 0.2 pt:
$$
\begin{array}{c|ccccc} \\
 & 0.8 & 0.9 & 1.0 & 1.1 & 1.2 \\
\hline
0.8 & 
  \asdf{X}{0.8}{0.8}{0.2pt} &
  \asdf{X}{0.8}{0.9}{0.2pt} &
  \asdf{X}{0.8}{1.0}{0.2pt} &
  \asdf{X}{0.8}{1.1}{0.2pt} &
  \asdf{X}{0.8}{1.2}{0.2pt} \\
0.9 & 
  \asdf{X}{0.9}{0.8}{0.2pt} &
  \asdf{X}{0.9}{0.9}{0.2pt} &
  \asdf{X}{0.9}{1.0}{0.2pt} &
  \asdf{X}{0.9}{1.1}{0.2pt} &
  \asdf{X}{0.9}{1.2}{0.2pt} \\
1.0 & 
  \asdf{X}{1.0}{0.8}{0.2pt} &
  \asdf{X}{1.0}{0.9}{0.2pt} &
  \asdf{X}{1.0}{1.0}{0.2pt} &
  \asdf{X}{1.0}{1.1}{0.2pt} &
  \asdf{X}{1.0}{1.2}{0.2pt} \\
1.1 & 
  \asdf{X}{1.1}{0.8}{0.2pt} &
  \asdf{X}{1.1}{0.9}{0.2pt} &
  \asdf{X}{1.1}{1.0}{0.2pt} &
  \asdf{X}{1.1}{1.1}{0.2pt} &
  \asdf{X}{1.1}{1.2}{0.2pt} \\
1.2 & 
  \asdf{X}{1.2}{0.8}{0.2pt} &
  \asdf{X}{1.2}{0.9}{0.2pt} &
  \asdf{X}{1.2}{1.0}{0.2pt} &
  \asdf{X}{1.2}{1.1}{0.2pt} &
  \asdf{X}{1.2}{1.2}{0.2pt}
\end{array}$$ 

Thickness 0.3 pt:
$$
\begin{array}{c|ccccc} \\
 & 0.8 & 0.9 & 1.0 & 1.1 & 1.2 \\
\hline
0.8 & 
  \asdf{X}{0.8}{0.8}{0.3pt} &
  \asdf{X}{0.8}{0.9}{0.3pt} &
  \asdf{X}{0.8}{1.0}{0.3pt} &
  \asdf{X}{0.8}{1.1}{0.3pt} &
  \asdf{X}{0.8}{1.2}{0.3pt} \\
0.9 & 
  \asdf{X}{0.9}{0.8}{0.3pt} &
  \asdf{X}{0.9}{0.9}{0.3pt} &
  \asdf{X}{0.9}{1.0}{0.3pt} &
  \asdf{X}{0.9}{1.1}{0.3pt} &
  \asdf{X}{0.9}{1.2}{0.3pt} \\
1.0 & 
  \asdf{X}{1.0}{0.8}{0.3pt} &
  \asdf{X}{1.0}{0.9}{0.3pt} &
  \asdf{X}{1.0}{1.0}{0.3pt} &
  \asdf{X}{1.0}{1.1}{0.3pt} &
  \asdf{X}{1.0}{1.2}{0.3pt} \\
1.1 & 
  \asdf{X}{1.1}{0.8}{0.3pt} &
  \asdf{X}{1.1}{0.9}{0.3pt} &
  \asdf{X}{1.1}{1.0}{0.3pt} &
  \asdf{X}{1.1}{1.1}{0.3pt} &
  \asdf{X}{1.1}{1.2}{0.3pt} \\
1.2 & 
  \asdf{X}{1.2}{0.8}{0.3pt} &
  \asdf{X}{1.2}{0.9}{0.3pt} &
  \asdf{X}{1.2}{1.0}{0.3pt} &
  \asdf{X}{1.2}{1.1}{0.3pt} &
  \asdf{X}{1.2}{1.2}{0.3pt}
\end{array}$$ 


Thickness 0.4 pt:
$$
\begin{array}{c|ccccc} \\
 & 0.8 & 0.9 & 1.0 & 1.1 & 1.2 \\
\hline
0.8 & 
  \asdf{X}{0.8}{0.8}{0.4pt} &
  \asdf{X}{0.8}{0.9}{0.4pt} &
  \asdf{X}{0.8}{1.0}{0.4pt} &
  \asdf{X}{0.8}{1.1}{0.4pt} &
  \asdf{X}{0.8}{1.2}{0.4pt} \\
0.9 & 
  \asdf{X}{0.9}{0.8}{0.4pt} &
  \asdf{X}{0.9}{0.9}{0.4pt} &
  \asdf{X}{0.9}{1.0}{0.4pt} &
  \asdf{X}{0.9}{1.1}{0.4pt} &
  \asdf{X}{0.9}{1.2}{0.4pt} \\
1.0 & 
  \asdf{X}{1.0}{0.8}{0.4pt} &
  \asdf{X}{1.0}{0.9}{0.4pt} &
  \asdf{X}{1.0}{1.0}{0.4pt} &
  \asdf{X}{1.0}{1.1}{0.4pt} &
  \asdf{X}{1.0}{1.2}{0.4pt} \\
1.1 & 
  \asdf{X}{1.1}{0.8}{0.4pt} &
  \asdf{X}{1.1}{0.9}{0.4pt} &
  \asdf{X}{1.1}{1.0}{0.4pt} &
  \asdf{X}{1.1}{1.1}{0.4pt} &
  \asdf{X}{1.1}{1.2}{0.4pt} \\
1.2 & 
  \asdf{X}{1.2}{0.8}{0.4pt} &
  \asdf{X}{1.2}{0.9}{0.4pt} &
  \asdf{X}{1.2}{1.0}{0.4pt} &
  \asdf{X}{1.2}{1.1}{0.4pt} &
  \asdf{X}{1.2}{1.2}{0.4pt}
\end{array}$$ 

Thickness 0.5 pt:
$$
\begin{array}{c|ccccc} \\
 & 0.8 & 0.9 & 1.0 & 1.1 & 1.2 \\
\hline
0.8 & 
  \asdf{X}{0.8}{0.8}{0.5pt} &
  \asdf{X}{0.8}{0.9}{0.5pt} &
  \asdf{X}{0.8}{1.0}{0.5pt} &
  \asdf{X}{0.8}{1.1}{0.5pt} &
  \asdf{X}{0.8}{1.2}{0.5pt} \\
0.9 & 
  \asdf{X}{0.9}{0.8}{0.5pt} &
  \asdf{X}{0.9}{0.9}{0.5pt} &
  \asdf{X}{0.9}{1.0}{0.5pt} &
  \asdf{X}{0.9}{1.1}{0.5pt} &
  \asdf{X}{0.9}{1.2}{0.5pt} \\
1.0 & 
  \asdf{X}{1.0}{0.8}{0.5pt} &
  \asdf{X}{1.0}{0.9}{0.5pt} &
  \asdf{X}{1.0}{1.0}{0.5pt} &
  \asdf{X}{1.0}{1.1}{0.5pt} &
  \asdf{X}{1.0}{1.2}{0.5pt} \\
1.1 & 
  \asdf{X}{1.1}{0.8}{0.5pt} &
  \asdf{X}{1.1}{0.9}{0.5pt} &
  \asdf{X}{1.1}{1.0}{0.5pt} &
  \asdf{X}{1.1}{1.1}{0.5pt} &
  \asdf{X}{1.1}{1.2}{0.5pt} \\
1.2 & 
  \asdf{X}{1.2}{0.8}{0.5pt} &
  \asdf{X}{1.2}{0.9}{0.5pt} &
  \asdf{X}{1.2}{1.0}{0.5pt} &
  \asdf{X}{1.2}{1.1}{0.5pt} &
  \asdf{X}{1.2}{1.2}{0.5pt}
\end{array}$$ 

\end{document}
