\documentclass[border=10pt]{standalone}
\usepackage{pgfplots,tikz,amsmath}
\usepackage{filecontents}
\usetikzlibrary{arrows.meta}
\usetikzlibrary{colorbrewer}
\usepgfplotslibrary{colorbrewer}
%\usepgfplotslibrary{colormaps}
%\usepackage{pgfplotstable}
\usepgfplotslibrary{external}
%\tikzexternalize[shell escape=-shell-escape]
\begin{document}

\begin{tikzpicture}
\begin{axis}[xlabel=$x$,
ylabel= $c(x)$,
width = 5.0 cm,
height = 3.0 cm,
transpose legend,
legend columns=-1,
%grid = both,
ylabel near ticks,
xlabel near ticks,
xmin=0.0,
xmax = 1.0,
%cycle list/RdGy-11,
cycle list/Paired-10,
%cycle list name=Paired-10,
%cycle list name=linestyles*,
]
\addplot+[mark = none, smooth, thick] table[x index=0,y index=1] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=25] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=49] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=73] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=97] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=121] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=145] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=169] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=193] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=217] {diffusion_19Jun.txt};
\end{axis}
\end{tikzpicture}

\begin{tikzpicture}
\begin{axis}[xlabel=$x$,
ylabel= $\varrho_a(x)$,
width = 5.0 cm,
height = 3.0 cm,
transpose legend,
legend columns=-1,
%grid = both,
ylabel near ticks,
xlabel near ticks,
xmin=0.0,
xmax = 1.0,
%cycle list/RdGy-11,
cycle list/Paired-10,
%cycle list name=Paired-10,
%cycle list name=Paired-5,
%cycle list name=linestyles*,
]
\addplot+[mark = none, smooth, thick] table[x index=0,y index=2] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=26] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=50] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=74] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=98] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=122] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=146] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=170] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=194] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=218] {diffusion_19Jun.txt};
\end{axis}
\end{tikzpicture}

\begin{tikzpicture}
\begin{axis}[xlabel=$x$,
ylabel= $\varrho_s(x)$,
width = 5.0 cm,
height = 3.0 cm,
transpose legend,
legend columns=-1,
%grid = both,
ylabel near ticks,
xlabel near ticks,
xmin=0.0,
xmax = 1.0,
%cycle list/RdGy-11,
cycle list/Paired-10,
%cycle list name=Paired-10,
%cycle list name=Paired-5,
%cycle list name=linestyles*,
]
\addplot+[mark = none, smooth, thick] table[x index=0,y index=3] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=27] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=51] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=75] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=99] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=123] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=147] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=171] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=195] {diffusion_19Jun.txt};
\addplot+[mark = none, smooth, thick] table[x index=0,y index=219] {diffusion_19Jun.txt};
\end{axis}
\end{tikzpicture}

\end{document}