title: jqplot problem with highlighter
body: <p>I'm using the jQuery plugins provided by jqplot for charting.
  When I use seriesDefaults : { fill : true }, the highlighter (tooltip) stops working.</p>
  
  <p>Here is the code I'm using.</p>
  
  <pre><code>var line1=[['23-May-08', 578.55], ['20-Jun-08', 566.5], ['25-Jul-08', 480.88], ['22-Aug-08', 509.84],
      ['26-Sep-08', 454.13], ['24-Oct-08', 379.75], ['21-Nov-08', 303], ['26-Dec-08', 308.56],
      ['23-Jan-09', 299.14], ['20-Feb-09', 346.51], ['20-Mar-09', 325.99], ['24-Apr-09', 386.15]];
  
  var plot = $.jqplot('chartarea', [line1],
                     {
                      seriesDefaults : { fill : true },
                      title          : "Area chart with tooltip",
                      axes           : {
                                        xaxis : { renderer : $.jqplot.DateAxisRenderer },
                                        yaxis : { tickOptions : { formatString : '$%.2f'} }
                                       },
                      highlighter    : { show : true, sizeAdjust : 7.5},
                      cursor         : { show : false } 
                     });
  </code></pre>
  
  <p>Is there any way to get the highlighter working in fill : true case.</p>
  
  <p>Thanks..</p>