Cette réponse est basée sur ma réponse de https://texnique.fr/osqa/questions/6218/legende-dun-graphique
\documentclass[10pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[french]{babel}
\usepackage[a4paper,left=10mm,right=10mm,top=20mm,bottom=20mm]{geometry}
%\usepackage{color}
\usepackage[dvipsnames]{xcolor}
\usepackage{tikz}
\usepackage{graphicx}
%\usepackage{picins}
\DeclareGraphicsExtensions{.jpg,.pdf,.png}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{amsfonts}
% \usepackage{siunitx}
% \usepackage{multicol}
\usepackage{url}
\usepackage{setspace}
\usepackage{gensymb}
\usepackage{mathrsfs}
\usepackage{chemfig}
\usepackage{pgfplots}
\usepackage{siunitx}
\usepackage{booktabs}
\usepackage{fancyhdr}
\pagestyle{fancy}
\usepackage{siunitx}
\usepackage{filecontents}
\begin{filecontents*}{test.txt}
X Intensity
1 2
2 4
4 2
8 1
\end{filecontents*}
\begin{document}
\begingroup
\centering
\begin{tikzpicture}
\begin{axis}[axis x line=bottom,axis y line = left,grid=major,
title={Taille des particules de F127a5 dans l'eau à $pH=2$ pour $c_{m} = \SI{20}{g.L^{-1}}$ à différentes températures},
xlabel={taille (nm)},
xmode=log,
ylabel={intensité (u.a.)},
ymax=63,
mark size=0.5pt,
width=1\textwidth,
%legend style={at={(0.5,-0.15)},anchor=north},
%legend columns=4,
% log ticks with fixed point
]
\addplot[purple,thick,smooth,x=X,y=Intensity] table[y expr={\thisrowno{1} + 0}] {test.txt};
\addplot[blue,thick,smooth,x=X,y=Intensity] table[y expr={\thisrowno{1} + 6}] {test.txt};
\addplot[green,thick,smooth,x=X,y=Intensity] table[y expr={\thisrowno{1} + 12}] {test.txt};
\addplot[red,thick,smooth,x=X,y=Intensity] table[y expr={\thisrowno{1} + 18}] {test.txt};
\legend{$T=20\celsius$,$T=30\celsius$,$T=40\celsius$,$T=50\celsius$};
\end{axis}
\end{tikzpicture}
\endgroup
\end{document}
![alt text][1]
[1]: https://texnique.fr/osqa/upfiles/Screen_Shot_2019-07-15_at_13.12.51.png
