I am trying to mimic the lxy example in

http://code.google.com/apis/chart/image/docs/gallery/line_charts.html#ax...

but I get no line (even if axes ranges are correct) and the following

warning: reset() expects parameter 1 to be array, null given in ...

function co2_it() 
{
 $canvas = charts_graphs_get_graph('google-charts');
 $canvas->parameters['cht'] = "lxy";
 $canvas->colour            = '#ffffff';

 $canvas->parameters['chxt'] = "x,y";
 $canvas->parameters['chxr'] = "0,2000,2009|1,0,700";
 $canvas->parameters['chd']  = "t:2000,2003,2004,2005,2006,2007,2008,2009|693,651,582,547,534,518,503,468";
 

 $out = $canvas->get_chart();
 return $out;
}
print co2_it();

What am I doing wrong?

Thanks for any hint,