plot 'heat-data.txt' matrix with image;

6   5   4   3   1   0
3   2   2   0   0   1
0   0   0   0   1   0
0   0   0   0   2   3
0   0   1   1   1   0
0   1   0   1   1   0

addplot3[raw gnuplot]
            gnuplot{
set view map;
plot 'heat-data.txt' matrix with image};

addplot3[raw gnuplot,surf,shader=flat]gnuplot{
plot 'heat-data.txt' matrix with image;
};

documentclass[tikz, crop]{standalone}
usepackage{tikz}
usepackage{pgfplots}
pgfplotsset{compat=newest}
pgfplotsset{plot coordinates/math parser=false}
begin{document}

begin{tikzpicture}

begin{axis}[
colorbar right,
colormap/viridis,
view={0}{90}
]

%addplot3[raw gnuplot]gnuplot{
%plot 'heat-data.txt' matrix with image;
%};

addplot3[raw gnuplot,surf,shader=flat]gnuplot{
plot 'heat-data.txt' matrix with image;
};

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