Allow underscore in PGFPlots tick labels read from table

Question

I'm trying to use the text data from a table column as the tick labels in a plot. The text data contains underscores, which makes LaTeX throw a Missing $ inserted error. I'd usually get around this by loading the underscore package, and that works fine if I provide the list of tick labels explicitly using xticklabels={A_B,B_C}. However, when I use the xticklabels from table={<table name>}{<column name>} syntax, the underscores are not printed and I get the Missing $ error again.

I looked at what the the xticklabels from table option does, and basically it just reads the specified table column, writes the content into a list, and then unpacks that list for each tick, using the following macro:

\makeatletter
\def\pgfplots@user@ticklabel@list@x{%
    \pgfplotslistselectorempty\ticknum\of\pgfplots@xticklabels\to\tick
    \tick
}
\makeatother

\pgfplots@xticklabels is the list that the labels were stored into, and \tick is just a macro that holds the label text for a single tick. Using \show\tick in that macro shows that it really just holds the plain text:

> \tick=macro:
->A_B.

Now ordinarily, that should just work. If I say \def\testtext{_} and then I print that text using another macro, say \def\printtest{\testtext} \printtest, the underscore shows up.

How can I use underscores in ticklabels specified using xticklabels from table without having to replace the underscores in the data table with \_?

\documentclass{article}

\usepackage{underscore}
\usepackage{pgfplots}
\usepackage{pgfplotstable}


%% For your convenience: This is the unaltered macro that prints
%% the tick label when "xticklabels from table" is used
\makeatletter
\def\pgfplots@user@ticklabel@list@x{%
    \pgfplotslistselectorempty\ticknum\of\pgfplots@xticklabels\to\tick
    \tick
}
\makeatother


\pgfplotstableread{
Label Value_A
A_B 1
B_C 2
}\datatable

\begin{document}

  \begin{tikzpicture}
    \begin{axis}[
            xtick=data,
            % xticklabels={A_B,B_C}, % <- this works
            xticklabels from table={\datatable}{Label} % <- this doesn't
        ]
     \addplot table[x expr=\coordindex]{\datatable};
    \end{axis}
  \end{tikzpicture}
\end{document}

Answer 1

Would you consider this as a valid answer? I cannot really test it now and it's too long for a comment. The idea is to "disable the math-mode function of _ while we read the data".

\catcode`\_=12 % category other
\pgfplotstableread{
Label Value
A_B 1
B_C 2
}\datatable
\catcode`\_=8 % category subscript

Answer 2

Here's a patch to the table reading commands:

\documentclass{article}
\usepackage[T1]{fontenc}

\usepackage{etoolbox}
% \usepackage{underscore} % load if explicit underscores have to be used
\usepackage{pgfplots}
\usepackage{pgfplotstable}

\makeatletter
% Patch the command that precedes reading the table
\patchcmd{\pgfplotstablereadpreparecatcodes@}
  {\pgfplotstableinstallignorechars}
  {\catcode`\_=13 \begingroup\lccode`~=`_ 
   \lowercase{\endgroup\let~}\textunderscore
   \pgfplotstableinstallignorechars}
  {}{}
% Patch the command that restores the category codes
\patchcmd{\pgfplotstablereadpreparecatcodes}
  {\noexpand}
  {\catcode\string`\noexpand\_=\the\catcode\string`\_ \noexpand}
  {}{}
\makeatother

%% For your convenience: This is the unaltered macro that prints
%% the tick label when "xticklabels from table" is used
\makeatletter
\def\pgfplots@user@ticklabel@list@x{%
    \pgfplotslistselectorempty\ticknum\of\pgfplots@xticklabels\to\tick
    \tick
}
\makeatother

\pgfplotstableread{
Label Value
A_B 1
B_C 2
}\datatable

\begin{document}

  \begin{tikzpicture}
    \begin{axis}[
            xtick=data,
            %xticklabels={A_B,B_C}, % <- this works with package underscore
            xticklabels from table={\datatable}{Label} % <- this works
        ]
     \addplot table[x expr=\coordindex]{\datatable};
    \end{axis}
  \end{tikzpicture}
\end{document}

The only way to get a real underscore independently of the output encoding is to make _ active and equivalent to \textunderscore. The example uses T1, but runs also with OT1.