Multiple pgfplots from the same table indices

Question

I have a 3-column datafile where the first column specifies the paramter and the other two shall be the x and y coordinates of the respective curve. How can I make sure that curves of different parameter aren't connected and can ideally be uniquely labeled and/or colored?

I know I could use gnuplot or split up the datafile in multiple columns/files as mentioned in a similar question but I have a few hundred different parameters so I was hoping pgfplots would either allow me to filter out for each plot the coordinates that don't match their parameter or directly plot this datafile with multiple curves.

e.g.

\documentclass[preview]{standalone}
    \usepackage{pgfplots}
    \pgfplotsset{compat=newest}

    \begin{filecontents}{data.dat}
        1 0   0.5
        1 1   1
        1 2   1.5
        2 0   0
        2 0.5 0.5
        2 1   0.75
        2 1.5 0.865
    \end{filecontents}

    \usepackage{tikz}
    \begin{document}
        \begin{tikzpicture}
            \begin{axis}[]
            \addplot table[x index=1, y index=2]{data.dat};
            \end{axis}
        \end{tikzpicture}
    \end{document}

Produces ,but I want

Answer 1

Here is a proposal. I've adapted this answer from How to read a specific portion of a data file in pgfplots.

Use discard if not={<-parameter-col-num->}{<-parameter-value->}.

You mention that you have a few hundred parameters, so you could use a foreach loop such as:

\foreach \N in {1,2}{
\addplot table[x index=1, y index=2,discard if not={0}{\N}]{data.dat};
}

See the bottom of this post for a larger example using 5 parameters.

MWE

\documentclass[margin=0.5cm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}

\begin{filecontents}{data.dat} % data format: | Parameter | X | Y |
        1 0   0.5
        1 1   1
        1 2   1.5
        2 0   0
        2 0.5 0.5
        2 1   0.75
        2 1.5 0.865
\end{filecontents}

% Adapted from https://tex.stackexchange.com/a/178632/128068
% Changed \thisrow{#1} to \thisrowno{#1}        
\pgfplotsset{
    discard if not/.style 2 args={
        x filter/.code={
            \edef\tempa{\thisrowno{#1}}
            \edef\tempb{#2}
            \ifx\tempa\tempb
            \else
                \def\pgfmathresult{inf}
            \fi
        }
    }
}

\begin{document}
    \begin{tikzpicture}
        \begin{axis}[]
        \addplot table[x index=1, y index=2,discard if not={0}{1}]{data.dat};
        \addplot table[x index=1, y index=2,discard if not={0}{2}]{data.dat};
        \end{axis}
    \end{tikzpicture}
\end{document}

Here is an example of foreach with a larger data file of 5 parameters:

\documentclass[margin=0.5cm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}

\begin{filecontents}{data.dat} % data format: | Parameter | X | Y |
        1 0   0.5
        1 1   1
        1 2   1.5
        2 0   0
        2 0.5 0.5
        2 1   0.75
        2 1.5  0.865
        3 0   1.5
        3 1   2.6
        3 2   1.9
        4 0   4
        4 0.5 3.7
        4 1    3.6
        5 1.5 4.865
        5 2 2.5
\end{filecontents}

% Adapted from https://tex.stackexchange.com/a/178632/128068
% Changed \thisrow{#1} to \thisrowno{#1}       
\pgfplotsset{
    discard if not/.style 2 args={
        x filter/.code={
            \edef\tempa{\thisrowno{#1}}
            \edef\tempb{#2}
            \ifx\tempa\tempb
            \else
                \def\pgfmathresult{inf}
            \fi
        }
    }
}

\begin{document}
    \begin{tikzpicture}
        \begin{axis}[]
        \foreach \N in {1,...,5}{
        \addplot table[x index=1, y index=2,discard if not={0}{\N}]{data.dat};
        }
        \end{axis}
    \end{tikzpicture}
\end{document}

Answer 2

It's not the perfect solution because it requires a change to the datafiles and misuses colormaps for curve colors. unbounded coords=jump as axis parameter will interrupt the plot when nan inf or -inf appears in the datafile (sec. 4.4.10 of the pgfplots manual)

So the curves can be separated using a line of NaNs whenever the parameter changes.

    1 2   1.5
    NaN NaN NaN
    2 0   0

The parameter can be color-coded using meta and colormaps

\addplot[mesh, point meta = \thisrowno{0}] table[x index=1, y index=2]{data.dat};

It is now a mesh plot, for colored points – instead or in addition – a scatter plot can be used.

The colormap will stretch over the range of parameters which can make single curves hard to differentiate. point meta = {mod(\thisrowno0,8)} will alternate the colors every 8 paramters.

Put together:

\documentclass[preview]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}

\begin{filecontents}{data.dat}
    1 0   0.5
    1 1   1
    1 2   1.5
    NaN NaN NaN
    2 0   0
    2 0.5 0.5
    2 1   0.75
    2 1.5 0.865
\end{filecontents}

\usepackage{tikz}
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[unbounded coords=jump]
        \addplot[mesh, scatter, point meta = {mod(\thisrowno0,8)}] table[x index=1, y index=2]{data.dat};
        \end{axis}
    \end{tikzpicture}
\end{document}

Answer 3

Not modifying the data file

This is more or less an advanced answer of Milo's answer, which is adapted to make all the computations in Lua -- when compiling with LuaLaTeX -- instead of in TeX together with the also already mentioned tip to read the whole table only once using \pgfplotstableread (from the pgplotstable package) instead of for each \addplot command. I have not tested if this really improves compilation speed, but would guess so.

Modifying the data file

In general I would recommend to restructure the data file. If all parameter modifications where calculated with the same amount of data points then you can have them all in one data file. Further depending if the computations were done at the same x coordinates you can have one x column and then all the y columns or pairwise. (Again, this only works if all columns have the same amount of rows. If they don't, you could fill the rest with NaNs.)

Or you can store each parameter variation in its own data file (of course.)

With that also "maintenance" is much easier, especially if you do not want to plot all the "hundreds of variations" in one plot -- which most likely also is not a good idea.

% used PGFPlots v1.16
    % (borrowed modified data file from Milo's answer
    %  <https://tex.stackexchange.com/a/448407/95441>)
    \begin{filecontents}{data.dat}
        % data format: | Parameter | X | Y |
        1 0   0.5
        1 1   1
        1 2   1.5
        2 0   0
        2 0.5 0.5
        2 1   0.75
        2 1.5  0.865
        3 0   1.5
        3 1   2.6
        3 2   1.9
        4 0   4
        4 0.5 3.7
        4 1    3.6
        5 1.5 4.865
        5 2 2.5
    \end{filecontents}
\documentclass[margin=5pt]{standalone}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
    % use this `compat' level or higher to make use of Lua for computations
    % when using LuaLaTeX
    \pgfplotsset{compat=1.12}
    \pgfplotstableread{data.dat}{\loadedtable}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        % because we know we want to filter data points
        % we don't need a warning for every filtered data point in the log
        filter discard warning=false,
    ]
        \foreach \i in {1,...,5}{
            \addplot table [
                x expr={
                    ifthenelse(
                        \thisrowno{0} == \i,
                        \thisrowno{1},
                        NaN
                    )
                },
                y index=2,
            ] {\loadedtable};
        }
    \end{axis}
\end{tikzpicture}
\end{document}