[pstricks] discontinuous graphs

[Markus]

Hi all!

Is there any solution to prevent pstricks from drawing the vertical lines in

y = floor(x) - x

automatically? Is there any possibility to detect where the function 
is not continuous?

Minimal example

\documentclass{article}
\usepackage{pstricks-add}
\usepackage{amsmath,amssymb}
\begin{document}
\[
a(x) =  \lfloor x \rfloor -x \qquad \text{mit} \quad \lfloor x 
\rfloor = \max\{n \in \mathbb{Z} \mid n \le x\}
\]
\begin{center}\psset{plotpoints=2000}
\begin{pspicture}(-5,-1.2)(5,1.2)
\psaxes{->}(0,0)(-5,-1.2)(5,1.2)
\uput{5pt}[270](5,0){$x$}
\uput{5pt}[0](0,1.2){$y$}
%
\psplot[linecolor=blue]{-5}{4.99}{x floor x sub}
\end{pspicture}
\end{center}
\end{document}

Thanks for any help in advance!

Markus