[Tex/LaTex] How to mark/label nth data point from file in pgfplots

nodes-near-coordspgfplots

\documentclass{standalone}

\usepackage{pgfplots}
\usepackage{pgfplotstable}
\pgfplotsset{compat=1.10}

\pgfplotstableread{
0.01    1.00
0.02    2.00
0.03    3.00
0.04    4.00
0.05    5.00
}\datatable

\begin{document}
\begin{tikzpicture}
\begin{axis}[ymin=0, ymax=6]
\addplot table  {\datatable}
node[pos=0.0, pin=above:"Unidentified peak"]{} % pos only allows for fractional value
node[pos=0.2, pin=above:"Bragg peak"]{} % pos only allows for fractional value
node[pos=1.0, pin=above:"my custom label"]{} % pos only allows for fractional value
;

\end{axis}
\end{tikzpicture}
\end{document}

UPDATE:
A slightly shorter version of the second solution in the answer by @Matthew Leingang:

\documentclass{standalone}

\usepackage{pgfplots}
\usepackage{pgfplotstable} % For \pgfplotstableread
\pgfplotsset{compat=1.10}

\pgfplotstableread{
0.01    1.00
0.011   2.00
0.012   3.00
0.013    4.00
0.05    5.00
}\datatable
% \pgfplotstableread{results.dat}\datatable

\pgfplotstablegetrowsof{\datatable}
\pgfmathsetmacro{\N}{\pgfplotsretval}  

\begin{document}
\begin{tikzpicture}
\begin{axis}[ymin=0, ymax=6]
  \addplot+[mark=none, forget plot] table {\datatable};
  \addplot+[only marks] table  {\datatable}
    % pos = (index-1)/(N-1)  (index starting from 1)
    node[pos=(1-1)/(\N-1), pin=right:first point (1/\N)]{}
    node[pos=(3-1)/(\N-1), pin=-45:third point (3/\N)]{}
    node[pos=(4-1)/(\N-1), pin=-45:end of linear behaviour (4/\N)]{}
;
\end{axis}
\end{tikzpicture}
\end{document}

enter image description here

Best Answer

I think the easiest way to do it is as you've done, keeping in mind that if there are N equally spaced points, the pos-ition of the n-th point is (n-1)/(N-1).

In your case you have 5 points and you want to mark the second, so it should be at position 0.25:

\documentclass{standalone}

\usepackage{pgfplots}
\usepackage{pgfplotstable} % For \pgfplotstableread

\pgfplotstableread{
0.01    1.00
0.02    2.00
0.03    3.00
0.04    4.00
0.05    5.00
}\datatable

\begin{document}
\begin{tikzpicture}
\begin{axis}[ymin=0, ymax=6]
\addplot table  {\datatable}
node[pos=0.0, pin=right:``first point'']{} 
node[pos=0.25, pin=above:``second point'']{} 
node[pos=1.0, pin=left:``last point'']{} 
;

\end{axis}
\end{tikzpicture}
\end{document}

sample code output

This will work as along as your points are evenly spaced. If they are not, you have two choices that I know of:

  1. calculate the relative position based on the point coordinates. If they are labeled x_1, x_2, ..., x_N, the n-th point is at position (x_n - x_1)/(x_N - x_1).

  2. Use a plot handler that only plots the points without interpolation, e.g., only marks. Then you can use fractional positions and the handler will “snap to” the point indexed by that fraction. So if there are five points pos=0.25 will snap to the second point regardless of whether it is really 1/4th of the way from the left to right.

If you have unevenly spaced points and you want to mark them, interpolate them, and label them, you can use a two-step approach. First, plot without marks with \addplot+[mark=none,forget plot] ... Then plot only marks and label with \addplot+[only marks] ... node[pos=0.25] .... Here is an example of that:

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.8}
\begin{document}

\begin{tikzpicture}[
    declare function={
        f(\x) = 2*(\x)^3 - 3*(\x)^2 - 12 *\x;
    }]
    \begin{axis}[
        width=\linewidth,
        xmin=-3.3,xmax=4,
        ymin=-21,ymax=21,
        xtick={-1,0.5,2},
        xticklabels={$-1$,$\frac12$,$2$},
        ytick=\empty,
        samples=50
    ]
        \addplot+[domain=-3:4,mark=none,forget plot] {f(x)};
        \addplot+[samples at={-1.8117,-1,0,0.5,2,3.3117},only marks] {f(x)}
            node[pos=0,
                pin={-88:$\left(\frac{1}{4}\left(3 - \sqrt{105}\right),0\right)$},
                ] {}
            node[pos=0.2,pin={$(-1,7)$}] {}
            node[pos=0.4,pin={15:$(0,0)$}] {}
            node[pos=0.6,pin={0:$\left(\frac12,-6\frac12\right)$}] {}
            node[pos=0.8,pin={90:$(2,-20)$}] {}
            node[pos=1.0,pin={95:$\left(\frac{1}{4}\left(3 + \sqrt{105}\right),0\right)$}] {}
        ;
    \end{axis}
\end{tikzpicture}
\end{document}

second sample code output

The forget plot command keeps the plot cycle from shifting so the second plot is styled exactly as the first one is. The second \addplot marks specific (unevenly spaced) points. Since there are six of them, the position of the n-th is 0.2n.

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