Using meta to add symbol to bar plot

Question

I'm having some trouble using meta in pgfplots. I want to add an asterisk above some of the bars to show significance. I've plotted my bar plots from a table and I've tried assigning a column of the table to correspond to meta but it doesn't seem to work and after a good search I can't find anyone who has answered this question...

Check out my code at the bottom. I would be super grateful for some help.

Thank you for your time.

\usepackage{graphicx}
\usepackage{pgfplots}
\usepackage{float}
\usepackage{tikz}
\usepackage{caption}
\begin{figure}[h]
\begin{subfigure}[b]{1\textwidth}
    \centering
        \begin{tikzpicture}[trim axis left, trim axis right]
\begin{axis}[
    legend pos=north west,
    xlabel= Monensin Treatment (hrs),
    ylabel= GAPDH Normalised Expression,
    bar width={1cm},
    ymin=0,
    ymax=8,
    title = {PENK Expression},
    width=12cm,
    height=6cm,
    ytick={0,0.5,1,1.5},
    ymin=0,
    ymax=1.5,
    xtick={1,2,3,4},
    xticklabels={%
        Ctrl,
        1,
        6,
        24
        },
    grid=none,
    ybar,  enlarge x limits={abs=1},
    ]

\addplot[
    fill=blue!25,
    draw=black,
    point meta=y,
    every node near coord/.style={inner ysep=5pt},
    error bars/.cd,
        y dir=both,
        y explicit,
        point meta=explicit symbolic,
] 
table [y error=error, point meta=meta] {
x   y       error    label  meta

1   1.0128   0.07    1 
2   0.371  0.055     2      ***
3   0.30   0.0941    3      
4   0.224  0.0348    4      
};
%\node [above, black, font=\Large] at (axis cs: 0.371) {$\ast$}
\draw ({rel axis cs:0,0}|-{axis cs:0,0}) -- ({rel axis cs:1,0}|-{axis cs:0,0});
\end{axis}
\end{tikzpicture}
\caption{}
\end{subfigure}
\\\vspace{0.5cm}
\begin{subfigure}[b]{1\textwidth}
    \centering
        \begin{tikzpicture}[trim axis left, trim axis right]
\begin{axis}[
    legend pos=north west,
    ylabel= Ct Mean,
    xlabel= Monensin Treatment (hrs),
    bar width={1cm},
    title = {GAPDH Control},
    width=12cm,
    height=6cm,
    ytick={0,10,20,30},
    ymin=0,
    ymax=30,
    xtick={1,2,3,4},
    xticklabels={%
        Ctrl,
        1,
        6,
        24
        },
    grid=none,
    ybar,  enlarge x limits={abs=1},
    ]

\addplot[
    fill=red!25,
    draw=black,
    point meta=y,
    every node near coord/.style={inner ysep=5pt},
    error bars/.cd,
        y dir=both,
        y explicit
] 
table [y error=error] {
x   y           error    label

1   20.45   4.152    1
2   23.54  0.368     2 
3   22.71   0.492    3
4   23.82  0.221     4
};

\draw ({rel axis cs:0,0}|-{axis cs:0,0}) -- ({rel axis cs:1,0}|-{axis cs:0,0});
\end{axis}
\end{tikzpicture}
\caption{}
\end{subfigure}
\label{fig:penkexpression}
\caption{}
\end{figure}

Answer 1

Did that code actually work for you without errors?

Anyways, the crucial thing you're missing is nodes near coords. It is this option that actually enables printing of a meta value above the bars.

Further:

• As you want to print something that isn't a number, and that you provide in the table, you need to add point meta=explicit symbolic, as you have done in the first axis, but you must not place it after /error bars/.cd.

• In the \addplot table options, you need to provide meta=<column name>, not point meta=<column name>.

• I had to add a {} in the empty cells in the meta column.

\documentclass{article}
\usepackage{pgfplots}
\usepackage{subcaption}
\begin{document}
\begin{figure}
\begin{subfigure}[b]{1\textwidth}
    \centering
        \begin{tikzpicture}[trim axis left, trim axis right]
\begin{axis}[
    legend pos=north west,
    xlabel= Monensin Treatment (hrs),
    ylabel= GAPDH Normalised Expression,
    bar width={1cm},
    ymin=0,
    ymax=8,
    title = {PENK Expression},
    width=12cm,
    height=6cm,
    ytick={0,0.5,1,1.5},
    ymin=0,
    ymax=1.5,
    nodes near coords,
    xtick={1,2,3,4},
    xticklabels={%
        Ctrl,
        1,
        6,
        24
        },
    grid=none,
    ybar,  enlarge x limits={abs=1},
    ]

\addplot[
    fill=blue!25,
    draw=black,
    point meta=explicit symbolic,
    every node near coord/.style={inner ysep=5pt},
    error bars/.cd,
        y dir=both,
        y explicit,     
] 
table [y error=error,meta=meta] {
x   y       error    label  meta
1   1.0128   0.07    1      {}
2   0.371  0.055     2      ***
3   0.30   0.0941    3      {}  
4   0.224  0.0348    4      {}
};
%\node [above, black, font=\Large] at (axis cs: 0.371) {$\ast$}
\draw ({rel axis cs:0,0}|-{axis cs:0,0}) -- ({rel axis cs:1,0}|-{axis cs:0,0});
\end{axis}
\end{tikzpicture}
\caption{}
\end{subfigure}
\\[0.5cm]
\begin{subfigure}[b]{1\textwidth}
    \centering
        \begin{tikzpicture}[trim axis left, trim axis right]
\begin{axis}[
    legend pos=north west,
    ylabel= Ct Mean,
    xlabel= Monensin Treatment (hrs),
    bar width={1cm},
    title = {GAPDH Control},
    width=12cm,
    height=6cm,
    ytick={0,10,20,30},
    ymin=0,
    ymax=30,
    xtick={1,2,3,4},
    xticklabels={%
        Ctrl,
        1,
        6,
        24
        },
    grid=none,
    ybar,  enlarge x limits={abs=1},
    nodes near coords
    ]

\addplot[
    fill=red!25,
    draw=black,
    point meta=explicit symbolic,
    every node near coord/.style={inner ysep=5pt},
    error bars/.cd,
        y dir=both,
        y explicit
] 
table [y error=error,meta=label] {
x   y           error    label
1   20.45   4.152    1
2   23.54  0.368     2 
3   22.71   0.492    3
4   23.82  0.221     4
};

\draw ({rel axis cs:0,0}|-{axis cs:0,0}) -- ({rel axis cs:1,0}|-{axis cs:0,0});
\end{axis}
\end{tikzpicture}
\caption{}
\end{subfigure}
\label{fig:penkexpression}
\caption{}
\end{figure}
\end{document}