documentclass[]{standalone}
usepackage{pgfplots,pgfplotstable}
pgfplotsset{compat=newest}

begin{filecontents}{data1.txt}
0 0

1 1
end{filecontents}


begin{filecontents}{data2.txt}
0 0
1 1
end{filecontents}

begin{document}
begin{tikzpicture}
begin{axis}

  addplot [color=black] table[x index =0, y index = 1,]   {./data1.txt};

  addplot [color=red,very thick,dashed] table[x index =0, y index = 1,]   {./data2.txt};

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