Problems displaying a chart created with jgraph


I've written a set of scripts that run under XenForo, now I wanted to add some spice by displaying some data as a chart. I took a look around and stopped by jgraph.

The result of the chart creation is loaded like this:

ECHO '<img src="chart.php">';
I used the example from this page:

When I run this example on the same server not under XenForo (I have different documentroots), it is displayed according the image on their homepage.

Is the script run from one of my scripts, the resulting image is 0*0 pixels in size. So my conclusion is that I have to change something to get it going in XenForo, or is it impossible? (A short explanation for a non-PHP programmer would be nice).