filtered ampersand result in a corrupted url

To get a graph embeeded in a drupal page and generated on the fly, I wrote something like:

// snip
echo " <img src=\"apath/createagraph.php?data=1&power=2\"> ";

where createagraph.php contain calls to jpgraph libraries to generate a graph, but the matter is that the above line is filtered out and all *&* occurences are replaced by *&* generating a mispelled url:

<img src="apath/createagraph.php?data=1&amp;power=2">

I guess the problem should lie in filter.module but I can't go more in deep with code details.

I apologize for my bad english but I would thank all this powerful drupal community