How can I add a horizontal line into a plot in case of I've changed the x-axis?

Question

Here are my MWE:

\documentclass{standalone}

\usepackage{pgfplots}
\pgfplotsset{compat=1.17}

\begin{document}

\pgfplotstableread{
x y error 
P53     0.21    0.07 
IgG    -0.16    0.06 
cTnI    0.03    0.04 
PSA    -0.22    0.05 
Myo    -0.05    0.03 
AFP    -0.09    0.02 
Serum   0.05    0.06 
CKMB    0.89    0.01 
}\datatable

\begin{tikzpicture}
\begin{axis}[
    ybar,
    bar width=14pt,
    symbolic x coords={P53,IgG,cTnI,PSA,Myo,AFP,Serum,CKMB},
    xlabel=xlabel,
    ylabel=xlabel,
    ]
    \addplot+ [   
    error bars/.cd,
    y dir=both,
    y explicit relative, 
    ] table [y error=error] {\datatable};
    %\addplot+ (P53,0) -- (CKMB,0);
  \end{axis}
\end{tikzpicture}


\end{document}

And here is my expected result:

PS: Can you tell me what's wrong with the x-tick?

Answer 1

With use of the extra x tick is simple:

\documentclass[margin=3.14159]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.17}

\begin{document}

\pgfplotstableread{%
x       y       error
P53     0.21    0.07
IgG    -0.16    0.06
cTnI    0.03    0.04
PSA    -0.22    0.05
Myo    -0.05    0.03
AFP    -0.09    0.02
Serum   0.05    0.06
CKMB    0.89    0.01
    }\datatable

    \begin{tikzpicture}
\begin{axis}[
    ybar,
    bar width=14pt,
    symbolic x coords={P53,IgG,cTnI,PSA,Myo,AFP,Serum,CKMB},
    xlabel=xlabel,
    ylabel=ylabel,
    extra y ticks=(0),  % <---
    extra y tick style={grid, % <---
                        grid style={thick, red}},
            ]
\addplot + [error bars/.cd,
            y dir=both, y explicit relative,
            ] table [y error=error] {\datatable};
\end{axis}
    \end{tikzpicture}
\end{document}

Answer 2

I suspect that pgfplots only used every third bar in order to fit the x tick labels. OTOH, reducing the font size by itself was not sufficient.

The left and right sides are best defined using (rel axis cs:) while the data points are defined using (axis cs:). It was easier to define named coordinates first then find the perpendiculars: (sw |- zero) and (ne |- zero).

\documentclass{standalone}

\usepackage{pgfplots}
\pgfplotsset{compat=1.17}

\begin{document}

\pgfplotstableread{
x y error 
P53     0.21    0.07 
IgG    -0.16    0.06 
cTnI    0.03    0.04 
PSA    -0.22    0.05 
Myo    -0.05    0.03 
AFP    -0.09    0.02 
Serum   0.05    0.06 
CKMB    0.89    0.01 
}\datatable

\begin{tikzpicture}
\begin{axis}[
    ybar,
    bar width=14pt,
    symbolic x coords={P53,IgG,cTnI,PSA,Myo,AFP,Serum,CKMB},
    xlabel=xlabel,
    ylabel=ylabel,
    xtick=data,
    x tick label style={font={\tiny}}
    ]
    \addplot+ [   
    error bars/.cd,
    y dir=both,
    y explicit relative, 
    ] table [y error=error] {\datatable};
    %\addplot+ (P53,0) -- (CKMB,0);
    \coordinate (zero) at (axis cs: P53,0);
    \coordinate (sw) at (rel axis cs: 0,0);
    \coordinate (ne) at (rel axis cs: 1,1);
  \end{axis}
  \draw[red] (sw |- zero) -- (ne |- zero);
\end{tikzpicture}
\end{document}

Answer 3

There are several ways to add the horizontal line. I think the simplest way to add the line is using an extra y tick, because this works independently of the data.

% used PGFPlots v1.17
\documentclass[border=5pt]{standalone}
\usepackage{pgfplotstable}
    \pgfplotsset{compat=1.17}
\begin{document}
\begin{tikzpicture}
        \pgfplotstableread{
            x      y        error
            P53     0.21    0.07
            IgG    -0.16    0.06
            cTnI    0.03    0.04
            PSA    -0.22    0.05
            Myo    -0.05    0.03
            AFP    -0.09    0.02
            Serum   0.05    0.06
            CKMB    0.89    0.01
        }\datatable
    \begin{axis}[
        ybar,
        bar width=14pt,
        xlabel=xlabel,
        ylabel=ylabel,
%        % --------------------------------------------------------------------
%        % (much simpler than using `symbolic coords` is ...
%        symbolic x coords={P53,IgG,cTnI,PSA,Myo,AFP,Serum,CKMB},
        table/x expr=\coordindex,
        xtick=data,
        xticklabels from table={\datatable}{x},
        xticklabel style={
            rotate=90,
        },
        % ... By that you don't have to repeat yourself if "x values" change.)
        % ---------------------------------------------------------------------
        % extra line at $y = 0$
        extra y ticks={0},
        extra y tick labels={},
        extra y tick style={
            grid=major,
            grid style={
                red,
            },
        },
    ]
        \addplot+ [
            error bars/.cd,
                y dir=both,
                y explicit relative,
        ] table [y error=error] {\datatable};
    \end{axis}
\end{tikzpicture}
\end{document}