JpGraph 在linux下无法显示图片

function _day30Act() {				$url = $_GET['url'];		$datax=array(02-17,02-18,02-19,02-20,02-21);		$datay=array(36178,48821,48421,39011,50863);		$namex = '最近30天';		$namey = '';		$this->line($datax, $datay, $namex, $namey, $url);	}	function line($datax, $datay, $namex='', $namey='', $url){		require_once ('pto/jpgraph/jpgraph.php');		require_once ('pto/jpgraph/jpgraph_line.php');		require_once ('pto/jpgraph/jpgraph_date.php');		//$datay = array(8320,9360,14956,17028,13060,15376,25428,16216,28548,18632,22724,28460); //填充的数据// 		$graph = new Graph(1030,200,"auto");    //创建画布		$graph = new Graph(500,200,"auto");    //创建画布		$graph->img->SetMargin(70,40,30,40); //设置统计图所在画布的位置，左边距50、右边距40、上边距30、下边距40，单位为像素		$graph->img->SetAntiAliasing();     //设置折线的平滑状态		//$graph->SetScale("textlin");     //设置刻度样式		$graph->SetScale('intint');		$graph->SetShadow();      //创建画布阴影		$graph->title->Set($url.'趋势图'); //设置标题		$graph->title->SetFont(FF_SIMSUN,FS_BOLD); //设置标题字体		$graph->SetMarginColor("#fff");    //设置画布的背景颜色为淡蓝色		$graph->yaxis->title->Set($namey);		$graph->yaxis->title->SetFont(FF_SIMSUN,FS_BOLD); //设置Y轴标题的字体		$graph->xaxis->SetPos("min");		$graph->yaxis->HideZeroLabel();		$graph->ygrid->SetFill(true,'#EFEFEF@0.5','#BBCCFF@0.5');		//$datax=array("1月","2月","3月","4月","5月","6月","7月","8月","9月","10月","11月","12月"); //X轴		$graph->xaxis->SetTickLabels($datax);     //设置X轴		$graph->xaxis->title->Set($namex);		$graph->xaxis->SetFont(FF_SIMSUN);     //设置X坐标轴的字体		$graph->xaxis->title->SetFont(FF_SIMSUN,FS_BOLD); //设置Y轴标题的字体		$graph->yaxis->scale->SetGrace(20);//设置y轴刻度值分辨率		$p1 = new LinePlot($datay);     //创建折线图对象		$p1->mark->SetType(MARK_FILLEDCIRCLE);    //设置数据坐标点为圆形标记		//$p1->value->Show();//在折线每个点上显示具体的值		$p1->value->SetFormat('%0.1f');		$p1->mark->SetFillColor("#40BBF3");     //设置填充的颜色		$p1->mark->SetWidth(3);      //设置圆形标记的直径为4像素		$p1->SetColor("#7CD0F7");      //设置折形颜色为蓝色		$p1->SetCenter();      //在X轴的各坐标点中心位置绘制折线		$graph->Add($p1);      //在统计图上绘制折线		$graph->Stroke();      //输出图像	}