How can I make the vertical line of certain rows thicker in LaTeX tables?

Question

I want to make a table and highlight a certain region in it using thick lines as shown below.

In my attempt to reproduce the above table, I have used the tabu package to make thick horizontal and vertical lines. However, there isn't any command in LaTeX to make thick vertical lines spanning given rows. Basically, I'm looking for a vertical version of \tabucline. Here's my code and output:

\begin{table}[h!]
\centering
\renewcommand{\arraystretch}{1.75}
\begin{tabu}{|[1.5pt]c|c|c|c|c|c|c|c|}
    \tabucline[1.5pt]{-}
    (-4, -4) & (-3, -4) & (-2, -4) & (-1, -4) & (0, -4) & (1, -4) & (2, -4) & (3, -4)    \\
    \hline
    (-4, -3) & (-3, -3) & (-2, -3) & (-1, -3) & (0, -3) & (1, -3) & (2, -3) & (3, -3)    \\
    \hline
    (-4, -2) & (-3, -2) & (-2, -2) & (-1, -2) & (0, -2) & (1, -2) & (2, -2) & (3, -2)    \\
    \hline
    (-4, -1) & (-3, -1) & (-2, -1) & (-1, -1) & (0, -1) & (1, -1) & (2, -1) & (3, -1)    \\
    \tabucline[1.5pt]{5-8}\hline
    (-4, 0) & (-3, 0) & (-2, 0) & (-1, 0) & (0, 0) & (1, 0) & (2, 0) & (3, 0)    \\
    \tabucline[1.5pt]{2-4}\hline
    (-4, 1) & (-3, 1) & (-2, 1) & (-1, 1) & (0, 1) & (1, 1) & (2, 1) & (3, 1)    \\
    \hline
    (-4, 2) & (-3, 2) & (-2, 2) & (-1, 2) & (0, 2) & (1, 2) & (2, 2) & (3, 2)    \\
    \hline
    (-4, 3) & (-3, 3) & (-2, 3) & (-1, 3) & (0, 3) & (1, 3) & (2, 3) & (3, 3)    \\
    \hline\tabucline[1.5pt]{1-1}
  \end{tabu}
\end{table}

Answer 1

• don't use tabu. It is buggy and not maintained
• instead it I suggest to use (relative new) package tabularray
• using it you can define all horizontal as well vertical lines in table preamble.

\documentclass{article}
\usepackage{tabularray}

\begin{document}
    \begin{table}[ht]
    \centering
\begin{tblr}{hlines = {0.1pt}, vlines = {0.1pt},
             hline{1} = 1pt, 
             hline{5}={5-Z}{1pt}, hline{6}={2-4}{1pt}, hline{Z}={1}{1pt},
             vline{1}={1pt}, 
             vline{2}={6-Z}{1pt}, vline{5}={5}{1pt}, vline{Z}={1-4}{1pt},
             colspec =  {*{8}{Q[c, $]}},
             }
    (-4, -4) & (-3, -4) & (-2, -4) & (-1, -4) & (0, -4) & (1, -4) & (2, -4) & (3, -4)    \\
    (-4, -3) & (-3, -3) & (-2, -3) & (-1, -3) & (0, -3) & (1, -3) & (2, -3) & (3, -3)    \\
    (-4, -2) & (-3, -2) & (-2, -2) & (-1, -2) & (0, -2) & (1, -2) & (2, -2) & (3, -2)    \\
    (-4, -1) & (-3, -1) & (-2, -1) & (-1, -1) & (0, -1) & (1, -1) & (2, -1) & (3, -1)    \\
   (-4, 0)   & (-3, 0)  & (-2, 0)  & (-1, 0)  & (0, 0)  & (1, 0)  & (2, 0)  & (3, 0)     \\
    (-4, 1)  & (-3, 1)  & (-2, 1)  & (-1, 1)  & (0, 1)  & (1, 1)  & (2, 1)  & (3, 1)     \\
    (-4, 2)  & (-3, 2)  & (-2, 2)  & (-1, 2)  & (0, 2)  & (1, 2)  & (2, 2)  & (3, 2)     \\
    (-4, 3)  & (-3, 3)  & (-2, 3)  & (-1, 3)  & (0, 3)  & (1, 3)  & (2, 3)  & (3, 3)     \\
  \end{tblr}
\end{table}
\end{document}

Note: above MWE is tested with tabularray package version 2022C.

Answer 2

Here is a solution with {NiceTabular} of nicematrix.

In this environment, the key hvlines draws all the rules.

However, it's still possible to draw whatever rule you want with Tikz by using the Tikz nodes created by nicematrix under the rows, columns and cells.

\documentclass{article}
\usepackage{nicematrix,tikz}

\begin{document}
\begin{table}[ht]
\centering
\begin{NiceTabular}[hvlines]{cccccccc}
    (-4, -4) & (-3, -4) & (-2, -4) & (-1, -4) & (0, -4) & (1, -4) & (2, -4) & (3, -4)    \\
    (-4, -3) & (-3, -3) & (-2, -3) & (-1, -3) & (0, -3) & (1, -3) & (2, -3) & (3, -3)    \\
    (-4, -2) & (-3, -2) & (-2, -2) & (-1, -2) & (0, -2) & (1, -2) & (2, -2) & (3, -2)    \\
    (-4, -1) & (-3, -1) & (-2, -1) & (-1, -1) & (0, -1) & (1, -1) & (2, -1) & (3, -1)    \\
    (-4, 0)  & (-3, 0)  & (-2, 0)  & (-1, 0)  & (0, 0)  & (1, 0)  & (2, 0)  & (3, 0)     \\
    (-4, 1)  & (-3, 1)  & (-2, 1)  & (-1, 1)  & (0, 1)  & (1, 1)  & (2, 1)  & (3, 1)     \\
    (-4, 2)  & (-3, 2)  & (-2, 2)  & (-1, 2)  & (0, 2)  & (1, 2)  & (2, 2)  & (3, 2)     \\
    (-4, 3)  & (-3, 3)  & (-2, 3)  & (-1, 3)  & (0, 3)  & (1, 3)  & (2, 3)  & (3, 3)     \\
\CodeAfter
  \tikz \draw [very thick] (1-|1) |- (9-|2) |- (5-|5) |- (4-|last) |- (1-|1) -- cycle ; 
\end{NiceTabular}
\end{table}
\end{document}

You need several compilations (because of the PGF/Tikz nodes).