Scatterplot from table with markers set by meta column value

Question

My input data is a txt file with a number of columns and rows. The last column value is always an integer taking a value of 0, 1, 2, or 3.

I need a scatterplot which has a different marker depending on that end value.

\documentclass{article}
\usepackage{pgfplots}
\usepackage{filecontents}
\begin{filecontents*}{data.txt}
12.3458,0.709423,0.018174,10.3177,0.031258,0.360285,0.071809,0
13.3458,0.709423,0.018174,10.3177,0.031258,0.360285,0.171809,0
7.88918,0.037782,0.010597,13.0123,0.027078,0.345659,0.070872,1
8.88918,0.037782,0.010597,13.0123,0.027078,0.345659,0.170872,1
3.29679,0.175776,0.012142,18.2475,0.031448,0.292123,0.141521,2
4.29679,0.175776,0.012142,18.2475,0.031448,0.292123,0.241521,2
3.94161,0.204657,0.002334,2.09774,0.011567,0.278266,0.113811,3
4.94161,0.204657,0.002334,2.09774,0.011567,0.278266,0.213811,3
\end{filecontents*}

\begin{document}

\begin{tikzpicture}
    \pgfplotstableread[col sep=comma] {data.txt}\thedata
    \begin{axis}[width=7cm,height=7cm]
        \addplot[scatter,
                 only marks,
%                visualization depends on={value \thisrow{index 7} \as \labela},
%                scatter/@pre marker code/.append style = {/tikz/mark=\labela},
                 ] table[x index=0,y index=6] {\thedata};
    \end{axis}
\end{tikzpicture}

\end{document}

Resulting in

When I uncomment the lines, no marks appear.

• I need to define the markers to use: e.g. x, o, sqaure, triangle There are only ever 4.
• Preferably, I need to set their colour (e.g. x is red, o is blue, square is green, triangle is black)
• This graph will be used many times in a document with different data.
• I would like to keep pgfplotstableread within the tikzpicture if possible.

p.187 of the pgfplots manual looks relevant but I can't get any further

There are some other posts that are very close to being helpful, but none of them convert an integer into a marker symbol:

in PGFPlots, use meta data for markers?

Metadata for different colours in PGFPlots

How to plot scatter points using pgfplots with color defined from table rgb values?

Answer 1

You can use it as follows: Don't use macro names starting with \the. That is a special case for TeX and might lead to mistakes that are very difficult to debug.

\documentclass{article}
\usepackage{pgfplotstable}

\begin{document}
\begin{tikzpicture}
\pgfplotstableread[col sep=comma,header=false]{
12.3458,0.709423,0.018174,10.3177,0.031258,0.360285,0.071809,0
13.3458,0.709423,0.018174,10.3177,0.031258,0.360285,0.171809,0
7.88918,0.037782,0.010597,13.0123,0.027078,0.345659,0.070872,1
8.88918,0.037782,0.010597,13.0123,0.027078,0.345659,0.170872,1
3.29679,0.175776,0.012142,18.2475,0.031448,0.292123,0.141521,2
4.29679,0.175776,0.012142,18.2475,0.031448,0.292123,0.241521,2
3.94161,0.204657,0.002334,2.09774,0.011567,0.278266,0.113811,3
4.94161,0.204657,0.002334,2.09774,0.011567,0.278266,0.213811,3
}\mydata

\begin{axis}[width=7cm,height=7cm]
\addplot+[scatter, only marks,
  scatter/classes={0={mark=square*,green},
                   1={mark=triangle*,black},
                   2={mark=o,blue},
                   3={mark=x,red}
                  },
  scatter src=explicit symbolic
  ] table[x index=0,y index=6,meta index=7] \mydata;
\end{axis}

\end{tikzpicture}

\end{document}