Using Labels from a File in PGFplot

Question

I am using LaTeX, TikZ and PGFplot in order to automatically generate several series of diagrams, one of which is comprised of bar charts. Each bar in each bar chart may have a unique label and height -- I have used PGFplot before and I know that I can read in coordinates from files, but would it be possible for me to create labels for each bar using textual data stored in a tab-separated file? If so, how would I do this? A simplified example of a file I might use is shown below.

# foo.dat
# Label | X-Position | Height

A    1    15
B    2    20
C    3    12
D    4    24

Thanks for your help!

Answer 1

Depending on whether you want to use the label in place of the x tick label, or as additional information to each bar, here are two approaches.

You can use the key xticklabels from table={<table or file name>}{<column name>} to specify which column to read the labels from. These will be displayed instead of the x-coordinates of the bars. Note that you should also use the option xtick=data (keyword "data" is an in-built) to make sure that ticks are only created in those positions where a bar is drawn.

If you want to keep the x tick labels, you can add the textual labels at the top of each bar by using nodes near coords. This will add a text node above each bar, which will contain the meta data for that point. By default, this is the y coordinate. You can specify which column to use for the meta data by using meta=<column name> as an option to the table keyword. Note that for text data, you have to provide the option point meta=explicit to the plot, otherwise numerical data is expected.

\documentclass{article}
\usepackage{pgfplots}
\usepackage{filecontents}

\begin{filecontents}{testdata.dat}
Label X-Position Height
A    1    15
B    2    20
C    3    12
D    4    24
\end{filecontents}
%
\begin{document}
\begin{tikzpicture}
\begin{axis}[
    ybar,
    xtick=data,% crucial line for the xticklabels directive
    ymin=0,
    xticklabels from table={testdata.dat}{Label}
]

\addplot table [
    x=X-Position,
    y=Height
] {testdata.dat};

\end{axis}
\end{tikzpicture}
%
\hspace*{0.5cm}
%
\begin{tikzpicture}
\begin{axis}[
    ybar,
    xtick=data,
    ymin=0
]

\addplot +[
    nodes near coords,
    point meta=explicit symbolic
]
    table [x=X-Position,
        y=Height,
        meta=Label
] {testdata.dat};

\end{axis}
\end{tikzpicture}
\end{document}