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我正在尝试在量表上显示 mysql 数据库中的最新速度。我已经尝试了很多东西，但量规不显示请任何可以帮助我...我的代码已附加并且 php 部分显示正确的值但不知道为什么量规不显示

<?php

$host="localhost"; // Host name
$username="root"; // Mysql username
$password=""; // Mysql password
$db_name="mysql"; // Database name
$tbl_name="gpsdb"; // Table name

// Connect to server and select database.
$con=mysql_connect("$host", "$username")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");

$data = mysql_query("SELECT speed FROM gpsdb WHERE DeviceId=1234 ORDER BY TIME DESC LIMIT 1")
or die(mysql_error()); 

while ($nt = mysql_fetch_assoc($data))
{
$speed = $nt['speed'];
 $jsonTable = json_encode($speed);
 echo $jsonTable;

}

?>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8"/>
<title>
  Google Visualization API Sample
</title>
<script type="text/javascript" src="//www.google.com/jsapi"></script>
<script type="text/javascript">
  google.load('visualization', '1', {packages: ['gauge']});
</script>
<script type="text/javascript">
  function drawVisualization() {
    // Create and populate the data table.
    var data = new google.visualization.DataTable(<?=$speed?>);

    // Create and draw the visualization.
    new google.visualization.Gauge(document.getElementById('visualization')).
        draw(data);
  }

  google.setOnLoadCallback(drawVisualization);
</script>
</head>
<body style="font-family: Arial;border: 0 none;">
<div id="visualization" style="width: 600px; height: 300px;"></div>
</body>
</html>
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