 \begin{center}
  \setlength{\unitlength}{0.5cm}
  \referencesystem(0,0)(1,-1)(1,1)

  \fbox{\begin{picture}(6,6)(-3,-3)
   \cartesiangrid(-3,-3)(3,3)
  \end{picture}}\qquad
  \fbox{\begin{Picture}(-3,-3)(3,3)
   \cartesiangrid(-3,-3)(3,3)
  \end{Picture}}
 \end{center}
