[pstricks] Using the "psplot"

[Chris Sangwin]

Dear Stephane,

I often use PSTricks to calculate mathematical functions, etc and then
plot them.

The LaTeX package `fp' is useful for performing calculations, and the
following example - very helpfuly supplied by DG a few months ago in reply
to a similar queary of my own - uses psplot.  The image produced is a
quadratic equation (line 36) projected using single point
perspective.  This *is* an ellipse - but using psellipse would have been
cheating wouldn't it!

The moral is - there is no need to write more than one piece of
code.  LaTeX is capable of calculating without recourse to external
packages, such as Mathematica.

Regards
Chris

------ LaTeX Code follows --------------
\documentclass[11pt]{article}

\usepackage{pst-plot}     % From PSTricks
\usepackage{multido}
\usepackage[nomessages]{fp}

\newwrite\DataFileA

\newcommand{\DataFileNameA}{MyCurveA.dat}

\pagestyle{empty}

\begin{document}

\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{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(\iXX,\iY)(\iX,\iY)}          % End of the horizontal grid

  \immediate\openout\DataFileA=\DataFileNameA

  \multido{\IRx=-90+1}{181}{%
    \FPeval{\Ry}{ (\IRx * \IRx) / 30.0 }   % Defines the quadratic
    \PerCalc{\IRx}{\Ry}
    \immediate\write\DataFileA{(\iX,\iY)}
    }

  \immediate\closeout\DataFileA

  \readdata{\MyCurve}{\DataFileNameA}
%  \dataplot[plotstyle=curve,showpoints=true]{\MyCurve}
  \dataplot[plotstyle=curve]{\MyCurve}

\end{pspicture}

\end{document}