currently i do not want to upgrade some sites to eZ 4, so I was looking for a solution for eZ 3. Do or did you use your extension with a current (3.8+) ez 3 site ?
For eZ 4: Is there already an extension, that makes use of the new Graph component? That means: Can one already use the component in the ez templates?
My opinion is that it will more interesting for you to develop an generic extension that you can plug in each of your websote to generate graphs. So the bust is to use the eZComponent Graph.
http://ezcomponents.org/docs/tutorials/Graph
However, it needs PHP5 and by the way eZPublish 4.0.x.
If you would like to integrate one of the other solutions in your eZPublish 3.x, it will certainly work, but it might be more complex.