Il grafico deve risultare così:
sito del plugn

L'ho girato un attimo,effettivamente non centra tanto,ma se il problema è che salvo nel database le date con i trattini e i due punti,allora devo cambiare il formato e quindi con javascript ha poco che vedere...comunque ho girato un attimo il codice:
Codice PHP:
<?php

define ("DB_HOST", "localhost:3306"); // set database host
define ("DB_USER", "oxoc_sirdhee"); // set database user
define ("DB_PASS","Farebury"); // set database password
define ("DB_NAME","oxoc_utentiregi"); // set database name

echo'<html><head>  
  <link rel="stylesheet" type="text/css" href="../plugin/jquery.jqplot.css" />
  <link rel="stylesheet" type="text/css" href="../plugin/examples.css" />
  
      
    <script language="javascript" type="text/javascript" src="../plugin/jquery.min.js"></script>
    <script language="javascript" type="text/javascript" src="../plugin/jquery.jqplot.min.js"></script>
  
    <script language="javascript" type="text/javascript" src="../plugin/jqplot.logAxisRenderer.js"></script>
    <script language="javascript" type="text/javascript" src="../plugin/jqplot.canvasTextRenderer.js"></script>
    <script language="javascript" type="text/javascript" src="../plugin/jqplot.canvasAxisLabelRenderer.js"></script>
    <script language="javascript" type="text/javascript" src="../plugin/jqplot.canvasAxisTickRenderer.js"></script>
    <script type="text/javascript" src="../plugin/jqplot.canvasTextRenderer.min.js"></script>
    <script type="text/javascript" src="../plugin/jqplot.dateAxisRenderer.min.js"></script>    
    <script language="javascript" type="text/javascript" src="../plugin/jqplot.barRenderer.js"></script>
    
    <script type="text/javascript" src="../plugin/jqplot.cursor.min.js"></script>
        <script type="text/javascript" src="../plugin/jqplot.highlighter.min.js"></script>
    

    
    <style type="text/css">
      .jqplot-point-label {white-space: nowrap;}
          .jqplot { margin: 70px;}
    </style>
    
        </head><body background="images/bgpagina.jpg">';


$link = mysql_connect(DB_HOST, DB_USER, DB_PASS) or die("Couldn't make connection.");
$db = mysql_select_db(DB_NAME, $link) or die("Couldn't select database");
$con=0;

$res = mysql_query("SELECT * FROM `tempreg` ORDER BY id DESC LIMIT 0,11",$link);

if($res && mysql_num_rows($res)>0){
        echo "<center><div id='graf'  style='height:97%; width:98.5%'></div></center>";
    echo'</body></html>';
    echo "<script>"; 
    echo "instem=new Array();indate=new Array();inora=new Array();inf=new Array();infor=new Array;";
    
     while($row=mysql_fetch_assoc($res)){  
        echo "instem[".$con."]"."=".floatval($row['valore']).";";
        echo "indate[".$con."]"."='".intval($row['giorno'])."';";
        echo "inora[".$con."]='".intval($row['orario'])."';";
        //echo "inf[".$con."]=''['+indate[".$con."]+' '+ inora[".$con."]+','+instem[".$con."] +']'';";
        //echo "alert(inf[".$con."]);";
        $con+=1;
     }
     $con-=1;
    
    

    echo"

$(document).ready(function(){    
      $.jqplot.config.enablePlugins = true;
     /* for(i=0;i<".$con.";i++)
           inf[i].push(parseFloat(indate[i]),' ',parseFloat(inora[i]),',',' ',parseFloat(instem[i]));
       line2=inf;*/
       //line2=[['2011-06-24 12:17:56', 31.25],['2011-06-24 11:17:45', 29.79],['2011-06-24 10:47:40', 29.3],['2011-06-23 23:50:32', 28.32],['2011-06-23 22:50:20', 28.81],['2011-06-23 19:49:54', 29.79],['2011-06-23 18:58:30', 28.32],['2011-06-23 18:28:25', 28.32],['2011-06-23 17:58:19', 28.32],['2011-06-23 17:28:14', 28.81]]
       line2='[';
       line2+='[\''+indate[0]+' '+(inora[0])+'\''+','+' '+instem[0]+'],';
       for(i=1;i<".$con.";i++){
       if(i!=".$con."-1)
           line2+='[\''+indate[i]+' '+inora[i]+'\''+','+' '+instem[i]+'],';
       else
          line2+='[\''+indate[i]+' '+inora[i]+'\''+','+' '+instem[i]+'],';
           }
       line2+=']';
       
       alert(line2);
    plot2 = $.jqplot('graf', [line2], {
    series: [{ 
            renderer: $.jqplot.OHLCRenderer,
            rendererOptions: {
                candleStick: true,
            }
        }], 
      axes: {
        xaxis: {
          renderer: $.jqplot.DateAxisRenderer,
          label: 'Date',
          labelRenderer: $.jqplot.CanvasAxisLabelRenderer,
          tickRenderer: $.jqplot.CanvasAxisTickRenderer,
              tickInterval: '30 minutes', 
             /* min:'2011-06-23 12:00:00',
              max:'2011-06-24 12:00:00',*/
              tickOptions:{formatString:'%Y-%m-%d %H:%M:%S',angle: -30,},      
        },
        
        yaxis: {
          label: 'Temperature',
          labelRenderer: $.jqplot.CanvasAxisLabelRenderer,
          min:'0',
          max:'40',
        }
        
      },
      
      highlighter:{show:true,showTooltipUnitPosition: false,followMouse: false,showTooltipGridPosition: false,},
        cursor:{show: true,zoom:true,showTooltip:false,}
    });

});

</script>";
} else{

     echo "<center>Valori Insufficenti</center>";

     }
     
die($query);
?>