无法显示图像“http：// localhost / KVL / showgraph”，因为它包含错误 - Jpgraph

Question

I am using PHP Jpgraph library to create a graph from arrays generated from a mysql query.

I have written this php code:

require_once('jpgraph/src/jpgraph.php');
require_once('jpgraph/src/jpgraph_bar.php'); 

foreach ($results as $key => $value){

$data[] = $value['num_rows'];//$data = array(740,902,647,208);
$data1[] = $value['department_name'];//$data1 = array('Epi', 'Dss', 'Bio', 'Clin');


}

 $graph = new Graph(450,200,"auto");
 $graph->SetScale("textlin");
 $graph->img->SetMargin(40,30,40,40);

 $graph->SetShadow();

  $graph->xaxis->SetTickLabels($data1);

  $bplot = new BarPlot($data);
  $bplot->SetFillColor("lightgreen"); 
  $bplot->value->Show();
  $bplot->value->SetFont(FF_ARIAL,FS_BOLD);
  $bplot->value->SetAngle(45);
  $bplot->value->SetColor("black","navy");

  $graph->Add($bplot);
  $graph->Stroke();

I expect a bargraph generated by jpgraph. However, i get a blank page with a message "The image 'http://localhost/KVL/showgraph" cannot be displayed because it contains errors. I have run php -m on my terminal and can see that GD extension is installed. I am running php 7.2.