pgfplots: Add main diagonal (identity line) without affecting the axis

Question

I would like to add a main diagonal to a plot that does not affect the axis. The line should start and end at the axis or beyond them. Furthermore, I would like to avoid xminand xmax, because I work with data tables. I tried relative coordinates rel axis cs, but this does not result in a main diagonal (slope 1 and offset 0). Here is a minimal working example to start with:

\documentclass{scrartcl}

\usepackage{pgfplots}
\pgfplotsset{compat=1.9}

\begin{document}

\begin{tikzpicture}
    \centering
    \pgfplotsset{width=0.9\columnwidth}
    \begin{axis}
        \addplot coordinates {(0,0.1)(1,2)(2,3)};
    \end{axis}
\end{tikzpicture}

\end{document}

Answer 1

Thanks for the hint to percuss's excellent answer, sudosensei! From his code I derived the following one:

\draw[red]
(axis cs:\pgfkeysvalueof{/pgfplots/xmin},\pgfkeysvalueof{/pgfplots/xmin}) -- 
(axis cs:\pgfkeysvalueof{/pgfplots/xmax},\pgfkeysvalueof{/pgfplots/xmax});

This solves my problem:

Complete code:

\documentclass{scrartcl}

\usepackage{pgfplots}
\pgfplotsset{compat=1.9}

\begin{document}

\begin{tikzpicture}
    \centering
    \pgfplotsset{width=0.9\columnwidth}
    \begin{axis}
        \addplot coordinates {(0,0.1)(1,2)(2,3)};
        \draw[red]
            (axis cs:\pgfkeysvalueof{/pgfplots/xmin},\pgfkeysvalueof{/pgfplots/xmin}) -- 
            (axis cs:\pgfkeysvalueof{/pgfplots/xmax},\pgfkeysvalueof{/pgfplots/xmax});
    \end{axis}
\end{tikzpicture}

\end{document}