pgfplots - A TeX package to draw normal and/or logarithmic plots directly in TeX in two and three dimensions with a user-friendly interface and pgfplotstable - a TeX package to round and format numerical tables. Examples in manuals and/or on web site.
The downsampling option each nth point={<n>} always keeps points 0, n, 2n, ..., but users may want to keep points k, n-k, 2n-k, ... (0 <= k < n). Hence a more general option is required and will be helpful.
In https://tex.stackexchange.com/q/649944 I gave an attempt that provides each nth point starting from={<n>}{<k>} which makes each nth point={<n>} equivalent to each nth point starting from={<n>}{0}.
Another approach, letting skip coords between index={<begin>}{<end>} alter \coordindex, seems dangerous and will cause backwards compatibility problems.
Example copied from my tex-sx answer. Sorry it's a bit lengthy.
```tex
\begin{filecontents}{pts.dat}
x y
1 0
0.5 0.866
-0.5 0.866
-1 0
-0.5 -0.866
0.5 -0.866
1 0
\end{filecontents}
\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\makeatletter
\pgfplotsset{
/pgfplots/each nth point starting from/.style 2 args={
/pgfplots/each nth point starting from*={x}{#1}{#2}
},
% #1: the filter name: x, y, z, or something exotic like hist/data
% #2: the N of 'each nth'
% #3: the starting coordindex
% Example:
% {x}{3}{0} will leave indices 0, 3, 6, ...
% {x}{3}{1} will leave indices 1, 4, 7, ...
/pgfplots/each nth point starting from*/.style n args={3}{
/pgfplots/#1 filter/.append code={%
\ifnum\coordindex=0
\begingroup
% do not pollute \pgfmathresult
\pgfkeys{/pgf/fpu=false}%
\pgfmathsetmacro\c@pgfplots@eachnthpoint@xfilter{int(Mod(-(#3),#2))}%
\pgfmath@smuggleone\c@pgfplots@eachnthpoint@xfilter
\endgroup
\edef\c@pgfplots@eachnthpoint@xfilter@cmp{#2}%
\else
\pgfplotsutil@advancestringcounter\c@pgfplots@eachnthpoint@xfilter
\fi
\ifx\c@pgfplots@eachnthpoint@xfilter@cmp\c@pgfplots@eachnthpoint@xfilter
\def\c@pgfplots@eachnthpoint@xfilter{0}%
\else
\let\pgfmathresult\pgfutil@empty
\fi
}
},
}
\makeatother
\begin{document}
\begin{tikzpicture}
\begin{axis}[no marks]
\addplot[ultra thick, dashed, lightgray] table {pts.dat};
\foreach \m in {1,...,3} {
\edef\x{
\noexpand\addplot[
each nth point starting from={3}{\m},
filter discard warning=false,
unbounded coords=discard
] table {pts.dat};
}
\x
}
\foreach \m in {1,2} {
\edef\x{
\noexpand\addplot[blue,
each nth point starting from={2}{\m},
filter discard warning=false,
unbounded coords=discard
] table {pts.dat} -- cycle;
}
\x
}
\end{axis}
\end{tikzpicture}
\end{document}
```
The downsampling option
each nth point={<n>}always keeps points 0, n, 2n, ..., but users may want to keep points k, n-k, 2n-k, ... (0 <= k < n). Hence a more general option is required and will be helpful.In https://tex.stackexchange.com/q/649944 I gave an attempt that provides
each nth point starting from={<n>}{<k>}which makeseach nth point={<n>}equivalent toeach nth point starting from={<n>}{0}.Another approach, letting
skip coords between index={<begin>}{<end>}alter\coordindex, seems dangerous and will cause backwards compatibility problems.Example copied from my tex-sx answer. Sorry it's a bit lengthy.
```tex \begin{filecontents}{pts.dat} x y 1 0 0.5 0.866 -0.5 0.866 -1 0 -0.5 -0.866 0.5 -0.866 1 0 \end{filecontents} \documentclass[border=5pt]{standalone} \usepackage{pgfplots} \pgfplotsset{compat=1.18} \makeatletter \pgfplotsset{ /pgfplots/each nth point starting from/.style 2 args={ /pgfplots/each nth point starting from*={x}{#1}{#2} }, % #1: the filter name: x, y, z, or something exotic like hist/data % #2: the N of 'each nth' % #3: the starting coordindex % Example: % {x}{3}{0} will leave indices 0, 3, 6, ... % {x}{3}{1} will leave indices 1, 4, 7, ... /pgfplots/each nth point starting from*/.style n args={3}{ /pgfplots/#1 filter/.append code={% \ifnum\coordindex=0 \begingroup % do not pollute \pgfmathresult \pgfkeys{/pgf/fpu=false}% \pgfmathsetmacro\c@pgfplots@eachnthpoint@xfilter{int(Mod(-(#3),#2))}% \pgfmath@smuggleone\c@pgfplots@eachnthpoint@xfilter \endgroup \edef\c@pgfplots@eachnthpoint@xfilter@cmp{#2}% \else \pgfplotsutil@advancestringcounter\c@pgfplots@eachnthpoint@xfilter \fi \ifx\c@pgfplots@eachnthpoint@xfilter@cmp\c@pgfplots@eachnthpoint@xfilter \def\c@pgfplots@eachnthpoint@xfilter{0}% \else \let\pgfmathresult\pgfutil@empty \fi } }, } \makeatother \begin{document} \begin{tikzpicture} \begin{axis}[no marks] \addplot[ultra thick, dashed, lightgray] table {pts.dat}; \foreach \m in {1,...,3} { \edef\x{ \noexpand\addplot[ each nth point starting from={3}{\m}, filter discard warning=false, unbounded coords=discard ] table {pts.dat}; } \x } \foreach \m in {1,2} { \edef\x{ \noexpand\addplot[blue, each nth point starting from={2}{\m}, filter discard warning=false, unbounded coords=discard ] table {pts.dat} -- cycle; } \x } \end{axis} \end{tikzpicture} \end{document} ``` 