Libchart for display a chart

i used to Libchart for display a chart. i got a error in this line $chart->render(DRUPAL_ROOT.$file_name); The error is "Call to undefined function imageftbbox() in ..sites/all/libraries/libchart/libchart/classes/view/text/Text.php on line 67"

This is my coding

$form['sample_table'] = array(
			                       '#theme' => 'table',
			                       '#header' => $header_result,   
			                       '#rows' => $fin,
			                       '#empty' => t('page is not Present'),
			                       '#prefix' => '<div id="box" class="table">',
			                       '#suffix' => '</div>',
			                       );
			                       
			                       
	$name=views_get_view_result('views'); 
	 if(!empty($name)){	                                 
      // Generation of picture 
      require_once (DRUPAL_ROOT."/sites/all/libraries/libchart/libchart/classes/libchart.php");
      $chart = new LineChart();
      $dataSet = new XYSeriesDataSet();
      
     	 $serie = new XYDataSet();
    	 $serie2 = new XYDataSet();
        foreach ($nameIDS as $key_index4=> $row_value4) {
        $serie->addPoint(new Point($row_value4, $fin['average']['data'][$key_index4]['data']));
        $serie2->addPoint(new Point($row_value4, $avgIDS[$key_index4]));
      }
      
       $dataSet->addSerie("Average1', $serie);
       $dataSet->addSerie('Average2', $serie2);
       
      $chart->setDataSet($dataSet);
      $chart->setTitle("Average");
      $chart->getPlot()->setGraphCaptionRatio(0.62);
      $file_name = "/sites/default/files/chart/".time().'.png';  
     <strong> $chart->render(DRUPAL_ROOT.$file_name);</strong>
     
      global $base_url;
      
      $form['picture_gen'] = array(
                                     '#type'=>"markup",
                                     "#markup" => '<img border="0" src='.$base_url.$file_name.' alt="Pulpit rock" width="600" height="250" />',
                                      '#prefix' => '<div id="box" class="table">',
                                      '#suffix' => '</div>',
                                     );