Les unités sont différentes. `\draw` utilise cm, `\pgfpoint` utilise pt :
\documentclass[french,]{scrartcl}
\usepackage{babel}
\usepackage{tikz}
\usepackage{xcolor}
\begin{document}
\begin{tikzpicture}[xscale=0.1]
\begin{tikzpicture}[xscale=0.04,every node/.style={scale=.5}]
\draw (0,-0.5) node {1950} ;
\draw (20,-0.5) node[red] {1960} ; % invisible ( (20,0)
\pgfplothandlerybarinterval
\pgfplotstreamstart
\pgfplotstreampoint{\pgfpoint{0cm}{10}}
\pgfplotstreampoint{\pgfpoint{20cm}{0}} % «1960» attendu là
\pgfplotstreamend
\pgfusepath{stroke}
\end{tikzpicture}
\begin{tikzpicture}[scale=1,every node/.style={scale=.5}]
\draw (0pt,-0.5) node {1950} ;
\draw (20pt,-0.5) node[red] {1960} ; % invisible ( (20,0)
\pgfplothandlerybarinterval
\pgfplotstreamstart
\pgfplotstreampoint{\pgfpoint{0}{10}}
\pgfplotstreampoint{\pgfpoint{20}{0}} % «1960» attendu là
\pgfplotstreamend
\pgfusepath{stroke}
\end{tikzpicture}
\end{document}
