[pstricks] \psxTick error

[Johan Ekh]

Hi all,
I'm trying to use \psxTick but I get "Undefined control sequence" when I run
LaTeX.
Can anyone tell me what I am doing wrong?

I've included the following packages:

\usepackage{pstricks,pstricks-add,pst-math,pst-plot,pst-3dplot}


This is my code:

\begin{figure}[hbt]
  \begin{center}
    \psset{xAxisLabel=,yAxisLabel=}
    \begin{psgraph}[ticks=none,Dx=2,Dy=2](0,0)(-5,-5)(5,5){0.5\textwidth}{!}
      \psxTick(1.5){$a^2$}
      \psplot[linewidth=1.25pt,algebraic]{-4.0}{4.0}{ 3*sin(3.141592*x/8.0)
}
    \end{psgraph}
  \end{center}
\end{figure


//Johan
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