comma as decimal separator in table doesn't work with lua backend.

[Pitigrilli]

Compiling the following example with lualatex (This is LuaHBTeX, Version 1.15.0 (TeX Live 2022) (format=lualatex 2023.1.23) 24 JAN 2023 18:08) ) results in an empty diagram.

Warning in the log-file: NOTE: coordinate (--,--,--) [--](was (1,2,0,9,--) [--]) has been dropped because of a coordinate filter.

\documentclass[]{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}  % works only with pdflatex from compat=1.12 to 1.18
%\pgfplotsset{compat=1.11} % works with lualatex
\begin{document}
\begin{tikzpicture}
\begin{axis}
\addplot table [
    x = x,
    y = y,
    /pgf/number format/read comma as period
] { 
    x   y
    1,2 0,9
};
\end{axis}
\end{tikzpicture}
\end{document}

[dbitouze]

It's worth pointing out that the issue doesn't arise with tikz's datavisualization library: the following MCE compiles like a charm with lualatex (LuaHBTeX, Version 1.17.0 (TeX Live 2023)).

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{datavisualization}
\begin{document}

\begin{tikzpicture}
\datavisualization [school book axes, visualize as line]
data [separator=\space] {
x y
0.5 0
1.5 1
2.5 1
3.5 0
};
\end{tikzpicture}

\pgfset{/pgf/number format/read comma as period}

\begin{tikzpicture}
\datavisualization [school book axes, visualize as line]
data [separator=\space] {
x y
0,5 0
1,5 1
2,5 1
3,5 0
};
\end{tikzpicture}
\end{document}

[muzimuzhi]

\pgfplotsset{compat=1.18}  % works only with pdflatex from compat=1.12 to 1.18
%\pgfplotsset{compat=1.11} % works with lualatex

That's because lua backend is used by default since 1.12 https://github.com/pgf-tikz/pgfplots/blob/9f4e2b14af77fd196d769e98dbc622d68dd8df88/tex/generic/pgfplots/pgfplots.code.tex#L4681-L4685 Setting \pgfplotsset{compat=1.18, lua backend=false} works too (by hiding the problem).

[muzimuzhi]

The root cause has been reported to pgf in https://github.com/pgf-tikz/pgf/issues/1263.

@dbitouze (in response to your comment in tex-sx) The problem is not uncovered in tikz examples because it's only pgfplots that turns on fpu and the un-documented library luamath under LuaTeX by default inside axis environment. In general tikzpicture doesn't even live with turned-on fpu, see https://github.com/pgf-tikz/pgf/issues/678.

For a very specific (hence maybe highly limited) workaround, try the patch to \pgfplots@LUA@survey@point below

% !TeX program = lualatex
\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}

\makeatletter
\def\pgfplots@LUA@survey@point{%
  \ifpgfmathparsenumber@comma@as@period
    % Assume each point is a single number without any math function calls. 
    % Only then is reading comma as period unambiguous.
    \edef\pgfplots@loc@TMPa{pgfplots.texSurveyPoint(%
      gsub("\pgfplots@current@point@x", ",", "."),%
      gsub("\pgfplots@current@point@y", ",", "."),%
      gsub("\pgfplots@current@point@z", ",", "."),%
      "\pgfplots@current@point@meta")}%
    \pgfplotsutil@directlua{%
      gsub = string.gsub
      \pgfplots@loc@TMPa
    }%
  \else
    \edef\pgfplots@loc@TMPa{pgfplots.texSurveyPoint(%
      "\pgfplots@current@point@x",%
      "\pgfplots@current@point@y",%
      "\pgfplots@current@point@z",%
      "\pgfplots@current@point@meta")}%
    \pgfplotsutil@directlua{\pgfplots@loc@TMPa}%
  \fi
  % increase \pgfplots@current@point@coordindex:
  \advance\c@pgfplots@coordindex by1
}%
\makeatother

\begin{document}
\begin{tikzpicture}
  \begin{axis}
    \addplot table [
      x = x,
      y = y,
      /pgf/number format/read comma as period
    ] { 
      x   y
      1,2 0,9
      1,4 1,3
    };
  \end{axis}
\end{tikzpicture}
\end{document}

Update: An answer to question https://tex.stackexchange.com/q/687857 is added, though I'm a little resistant to posting experimental patch to different places. Hope it provides some more (user-friendly?) info.