Coordinate relative con pgfplots dentro ambiente axis

Perché le coordinate relative date dentro l’ambiente axis non funzionano?

`\documentclass{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[italian]{babel}
\usepackage{lmodern}
\usepackage[output-decimal-marker={,}]{siunitx}
\usepackage{pgfplots}

\pgfplotsset{/pgf/number format/use comma,compat=newest}
\begin{document}

\begin{tikzpicture}
\begin{axis}[xmin=0,xmax=12,ymin=-.2,ymax=0.2,no marks,grid,clip=false]
\addplot coordinates {(0,0) (2,.2)};
\draw (0,0) — ++(2,0.2);
\end{axis}
\end{tikzpicture}
\end{document}`

Ciao
Francesco