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}

This looks similar: http://tex.stackexchange.com/questions/55718/how-can-i-add-a-zero-line-to-a-plot. You can use axiscs in combination with \pgfkeysvalueof{/pgfplots/xmin} and variants to produce the main diagonal. I believe this will only work if you draw the line after your plot your data. — sudosensei, Nov 29 '13 at 09:40
By 'main diagonal', do you mean a line with slope 1 passing through the origin? — Jake, Nov 29 '13 at 10:38
@sudosensei: It is similar, but I do not want to specify xmin and xmax, because my data comes from tables that can change. @Jack Yes, slope 1 and no offset. — Dirk, Nov 29 '13 at 11:01
Oh, I misunderstood the question. Apologies. I have deleted my answer. You are in good hands with Jake. He's helped me countless times so far. :-) — sudosensei, Nov 29 '13 at 11:08
No problem. However, your answer gives me the correct hint to solve the problem, as you see in my answer. — Dirk, Nov 29 '13 at 14:27

score 7 · Accepted Answer · edited Apr 13 '17 at 12:35

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: enter image description here

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}

For the sake of completeness: Use \addlegendimage{line legend, red}\addlegendentry{Theory} to add a legend to the red line. — Dirk, Nov 30 '13 at 09:56

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

1 Answers1