How can I get correct picture?

I used Geogebra to draw the graph of the function $y = 2x^2 - 4x$. This is my code

\documentclass[10pt]{article}
\pagestyle{empty}
\begin{document}
\psset{xunit=1.0cm,yunit=1.0cm,algebraic=true,dotstyle=o,dotsize=3pt 0,linewidth=0.8pt,arrowsize=3pt 2,arrowinset=0.25}
\begin{pspicture*}(-2,-2.44)(3.92,5)
\psaxes[labelFontSize=\scriptstyle,xAxis=true,yAxis=true,Dx=1,Dy=1,ticksize=-2pt 0,subticks=2]{->}(0,0)(-2,-2.44)(3.92,5)[,140] [y,-40]
\psplot[linewidth=1.6pt,plotpoints=200]{-2.0}{3.9165217391304363}{2*x²-4*x}
\psline[linewidth=1.6pt,linestyle=dashed,dash=3pt 3pt](1,-2)(1,0)
\psline[linewidth=1.6pt,linestyle=dashed,dash=2pt 2pt](1,-2)(0,-2)
\begin{scriptsize}
\psdots[dotstyle=*,linecolor=blue](1,-2)
\rput[bl](1.02,-1.93){\blue{$A$}}
\psdots[dotstyle=*,linecolor=blue](1,0)
\rput[bl](1.02,0.08){\blue{$B$}}
\psdots[dotstyle=*,linecolor=blue](0,-2)
\rput[bl](0.02,-1.93){\blue{$C$}}
\end{scriptsize}
\end{pspicture*}
\end{document}


I use Texmaker and compiler with keys F2 -> F4 ->F8 ->F7. But I got

How can I get correct picture?

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– Louis Dec 18 '13 at 7:11

the function is not correct:

\psplot[linewidth=1.6pt,plotpoints=200]{-2.0}{3.9165217391304363}{2*x^2-4*x}


you have {2*x²-4*x}. An exponent must be written as x^2

However, the export of GeoGebra is not really nice. Use:

\documentclass[10pt]{article}
\pagestyle{empty}
\begin{document}

\psset{algebraic,linewidth=0.8pt,arrowsize=3pt 2,arrowinset=0.25}
\begin{pspicture*}(-2,-2.44)(3.92,5)
\psaxes[labelFontSize=\scriptstyle,ticksize=-2pt 0,subticks=2]{->}(0,0)(-2,-2.44)(3.92,5)[,140] [y,-40]
\psplot[linewidth=1.6pt,plotpoints=200]{-2.0}{3.9165217391304363}{2*x^2-4*x}
\psCoordinates[linecolor=blue,linestyle=dashed](1,-2)
\psdots[dotstyle=*,linecolor=blue](1,0)(0,-2)
\scriptsize
\rput[bl](1.02,-1.93){\blue{$A$}}
\rput[bl](1.02,0.08){\blue{$B$}}
\rput[bl](0.02,-1.93){\blue{$C$}}
\end{pspicture*}

\end{document}


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A recommended solution for best practitioners.

\documentclass[pstricks,border=12pt,12pt]{standalone}
\usepackage{pst-plot}
\usepackage{pst-eucl}

\psset
{
algebraic,
arrowsize=3pt 2,
arrowinset=0.25,
plotpoints=200,
saveNodeCoors,
}

\def\func(#1){2*(#1)^2-4*(#1)}

\begin{document}
\begin{pspicture}(-2,-2.5)(4,5)
\psplot[linecolor=red,yMaxValue=4]{-2.0}{4}{\func(x)}
\psaxes[labelFontSize=\scriptstyle,ticksize=-4pt 0,subticks=2,linecolor=lightgray]{->}(0,0)(-2,-2.5)(3.5,4.5)[$x$,0][$y$,90]
\everypsbox{\scriptsize\color{blue}}
\pstGeonode[linecolor=blue,PosAngle={90,-90,-135}](1,0){B}(*N-B.x {\func(x)}){A}(0,|A){C}
\psline[linestyle=dashed,linecolor=blue](B)(A)(C)
\end{pspicture}
\end{document}


Edit

I just knew by a trial and error that (x,y|P) can be simplified as (x,|P). And its dual, (P|x,y) can be simplified as (P|,y). I don't know whether it is a useful bug or a by-design feature.

Be careful, the yMaxValue apparently depends on plotpoints. A pessimistic approach is to use a large plotpoints (for example 5000) to make a better resolution, but the file size becomes bigger. :-)

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