Regression on a portion of the graph with columns (at the beginning)

Question

Based on the MWE of this question, I want to apply the regression only to a portion of the graph as in this other question, but not just the last portion. I want to do regression on the beginning so that the instruction skip first n=3 do not apply.

I can't make it work as I wish, i.e. regression only on the initial part of the graph. The command select coords between index={1}{3} generates an error.

\begin{tikzpicture} 
\begin{axis}[ytick={1,...,6}] 
\pgfplotstableread{% 
X Z Y 
1 3 0 
2 4 1 
3 5 2 
4 6 6 }\test 
\addplot[only marks, mark=*] 
     table[x=X, y=Y] {\test}; 
\addplot[   no markers,dashed, red, 
     select coords between index={1}{3}     
     ]      
     table [
     header=false,
     x=X,
     y={create col/linear regression={y=Y}}] {\test}; 
\end{axis}     
\end{tikzpicture}

When commenting the line with select coords between index the regression applies to all graph, and this is not what I want

Answer 1

As stated already a while ago in the comment below the question there is currently no way to ignore lines in the middle or the end while reading or processing a data table (in PGFPlots directly).

But you can use the raw gnuplot feature as a workaround to achieve that. (Of course that means that you have to have gnuplot installed on your machine).

Please have a look at the comments in the code for more details.

% used PGFPlots v1.14 and gnuplot v5.0 patchlevel 3
% (inspired by my solution given at <https://tex.stackexchange.com/a/343735/95441>)
\begin{filecontents*}{test.txt}
X Z Y
1 3 0
2 4 1
3 5 3
4 6 6
\end{filecontents*}
\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
\begin{document}
    \begin{tikzpicture}
        \begin{axis}
            \addplot [only marks, mark=*] table [x=X, y=Y] {test.txt};
            \addplot [dashed,red,] gnuplot [raw gnuplot] {
                % define the function to fit
                f(x) = a*x + b;
                % fit a and b by `using' columns 1 and 3 of 'test.txt' and
                % from that only the lines 1 to 3 (with the `every' key)
                fit f(x) 'test.txt' using 1:3 every ::1::3 via a, b;
                % set number of samples to 2, which is sufficient for a straight line
                set samples=2;
                % then return the resulting table to PGFPlots to plot it
                % using the x interval 1 to 4
                plot [x=1:4] f(x);
            };
             \end{axis}
    \end{tikzpicture}
\end{document}