title: jqPlot - How to change opacity or z-index of canvasOverlay?
body: <p>I would like to show 3 color zones on my graph on the background according to y axis value, as I understand, I cannot control the background color by different colors.</p>
  
  <p>My idea is to draw 3 horizontal lines with canvasOverlay - that is working.
  The problem is I want to place this lines behind my graph curve, now it seen on the front and it overlays my points line.</p>
  
  <p>Can I change the property of z-index or the opacity?</p>
  
  <p>Maybe some other ideas?</p>
  
  <pre><code>  $.jqplot( 'ChartDIV', [data],
          {
              series: [{ showMarker: true}],
              highlighter: {
                  sizeAdjust: 10,
                  show: true,
                  tooltipLocation: 'n',
                  useAxesFormatters: true
              },
  
              tickOptions: {
                  formatString: '%d'
              },
              canvasOverlay: {
                  show: true,
                  objects: [ 
                              {
                                  horizontalLine: 
                                  {      
                                      name: 'low', 
                                      y: 1.0,
                                      lineWidth: 100,
                                      color: 'rgb(255, 0, 0)',
                                      shadow: false 
                                  }
                              },      
                              {
                                  horizontalLine:
                                  { 
                                      name: 'medium',
                                      y: 2.0,
                                      lineWidth: 100, 
                                      color: 'rgb(250, 250, 0)', 
                                      shadow: true 
                                  }
                              },
                              {
                                   horizontalLine:
                                  {
                                      name: 'high',
                                      y: 3.0,
                                      lineWidth: 100,
                                      color: 'rgb(145, 213, 67)',
                                      shadow: false
                                  }
                               },  
                          ]       
                      },
              axes: {
                  xaxis:
                  {
                      label: 'Dates',
                      renderer: $.jqplot.DateAxisRenderer,
                      rendererOptions: { tickRenderer: $.jqplot.CanvasAxisTickRenderer },
                      tickOptions: {
                          formatString: '%d/%m/%Y',
                          angle: -30,
                          fontFamily: 'Arial',
                          fontSize: '13px',
                          fontWeight: 'bold'
                      },
                      min: d[0] + "/" + d[1] + "/01", 
                      tickInterval: '2 month',
                      labelOptions: {
                          fontFamily: 'Arial',
                          fontSize: '14pt',
                          fontWeight: 'bold',
                          textColor: '#0070A3'
                      }
                  },
                  yaxis:
                  {
                      label: 'Level',
                      labelRenderer: $.jqplot.CanvasAxisLabelRenderer,
                      tickOptions: {
                          formatter: $.jqplot.tickNumberFormatter
                      },
                      rendererOptions: { tickRenderer: $.jqplot.CanvasAxisTickRenderer },
                      labelOptions: {
                          fontFamily: 'Arial',
                          fontSize: '14pt',
                          fontWeight: 'bold',
                          textColor: '#0070A3',
                          angle: -90
                      }
  
                  }
              }
          } );
  </code></pre>