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

Question

我正在使用PHP Jpgraph库从mysql查询生成的数组创建图形。

我已经编写了以下php代码：

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();

我希望jpgraph生成一个条形图。但是，我得到一个空白页，并显示一条消息“图像'http://localhost/KVL/showgraph”，因为其中包含错误，所以无法显示。我已经在终端上运行php -m，可以看到已安装GD扩展。我正在运行php 7.2。