\documentclass[border=1mm]{standalone}

\usepackage{cmap}
\usepackage[cp1250]{inputenc}	
\usepackage[T1]{fontenc}
\usepackage{graphicx}
\usepackage{lmodern}
\usepackage{tikz}
\usepackage{float}
\usepackage{siunitx}
\usepackage{amsmath}
\usepackage{upgreek}
\usepackage{pgfplots}
\pgfplotsset{compat=1.17}
\usepgfplotslibrary{units}
% \makeatletter
%\pgfplotsset{
%  unit code/.code 2 args=
%    \begingroup
%    \protected@edef\x{\endgroup\si{#2}}\x
%} 
%\makeatother

\pgfkeys{%
  /tikz/on layer/.code={
    \pgfonlayer{#1}\begingroup
    \aftergroup\endpgfonlayer
    \aftergroup\endgroup
  }
}

\usepackage{csquotes}

\begin{document}
	\begin{tikzpicture}
		\begin{axis}[
			height=7cm,	width=11cm,	grid=major,
    		x unit=\SI{}{\micro\meter}, y unit=-,
			xlabel=$\lambda$,	ylabel=Transmittance,
			ymin=0,ymax=1,
			xmin=0.8, xmax=2.2,
			%ytick distance = 10,
			set layers,
			%x tick label style={/pgf/number format/.cd, set thousands separator={}},
			xticklabel shift = 3 pt
		]					
			
			\addplot [mark=none,color=blue,thick, each nth point={50}] table [x index=1, y index=2] {atmosphere_0u8_to_2u2.dat};
			
		\end{axis}
	\end{tikzpicture}
\end{document}