\documentclass{standalone}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\pgfplotsset{compat = 1.13}
\pgfplotstableset{col sep = comma}

\begin{document}
\pgfplotstableread{
20, 1000
70, 500
}{\data}

\begin{tikzpicture}
\begin{axis}

\addplot[black] table[x index = 0, y index = 1, col sep = comma] {\data};
\addlegendentry[]{040\_Mn}
\addplot[red!20!blue] table[x index = 0, y expr={\thisrowno{1} - 500}] {\data};
\addlegendentry[]{023\_Mn}
\addplot[yshift=-2cm,red!20!blue] table[x index = 0, y index=1] {\data};
\end{axis}
\end{tikzpicture}

\end{document}