\documentclass[12pt]{article} % Chris Sangwin 2004-06-23
\usepackage{pstricks} 
\usepackage{pst-plot}  
\usepackage{multido}
\usepackage[nomessages]{fp}
\pagestyle{empty}
\parindent=0pt
%
\newwrite\DataFileA
\newwrite\DataFileB
%
\newcommand{\DataFileNameA}{MyCurveA.dat}
\newcommand{\DataFileNameB}{MyCurveB.dat}
%
\def\Xpsz{6}
\def\Ypsz{6}

\newcommand{\PerCalc}[2]{%
  \def\ix{#1}\def\iy{#2}%
  \FPeval{\iY}{\Ypsz * \iy / (\iy + 1.0)}  % Calculates Y-coordinate
  \FPeval{\iX}{(\Ypsz - \iY)* \ix / \Xpsz} % Calculates X-coordinate
}% End perspective calculate

\begin{document}

\begin{pspicture}(-6,0)(6,6) 
  \multido{\I=-6+1}{13}{\psline(0,6)(\I,0)}

  % The horizontal perspective grid
  \multido{\IR=0+1}{13}{%
    \PerCalc{-6.0}{\IR}
    \FPneg{\iXX}{\iX}                         % Exploits symmetry 
    \psline[linecolor=red](\iXX,\iY)(\iX,\iY)}% End of the horizontal grid

  \immediate\openout\DataFileA=\DataFileNameA
  \immediate\openout\DataFileB=\DataFileNameB

  \multido{\IRx=0+1}{90}{%
    \FPeval{\Ry}{ (\IRx * \IRx) / 30.0 }   % Defines the quadratic
    \PerCalc{\IRx}{\Ry}
    \immediate\write\DataFileA{(\iX,\iY)}
    \FPneg{\iXX}{\iX} % Exploits symmetry - so we'll have to loose this...
    \immediate\write\DataFileB{(\iXX,\iY)}}

  \immediate\closeout\DataFileA
  \immediate\closeout\DataFileB

  \readdata{\MyCurve}{\DataFileNameA}
  \dataplot[plotstyle=curve,%
	showpoints=true,%
	linecolor=green,linewidth=1.5pt]{\MyCurve}

  \readdata{\MyCurve}{\DataFileNameB}
  \dataplot[plotstyle=curve,%
	showpoints=true,%
	linecolor=blue,linewidth=1.5pt]{\MyCurve}
\end{pspicture}

\end{document}

