gistfile1.txt

\documentclass{article}
\usepackage{siunitx}
\usepackage{etoolbox}
\robustify\bfseries
\begin{document}

\sisetup{
	retain-explicit-plus,
	detect-all = true
}

\begin{table}[h]
  \centering
    \begin{tabular}{ccc S[table-format=1.1, table-text-alignment=center]}                                      
		1.2 & 9.7 &  & \bfseries +1.3 \\
        1.9 & 0.7 &  & -2.0 \\
        1.9 & 0.7 &  & \large -2.0 \\        
        8.4 & 7.3 &  & +1.1
    \end{tabular}
\end{table}

\end{document}