I have a similar issue, using PHP and the FusionCharts js class.  I create the chart with code like this: 
  
 
var myChart = new FusionCharts("Charts/Gantt.swf", "myChartId", "1000", "100", "0", "0");
myChart.setDataURL("index_chart.php?id=1&subId=2");
 
  
But in chart.php, $_GET contains only id, not subId.