IslandPlot track and plot stacking

data.js

var tracks = [
   {
      trackName: "track1",
      trackType: "track",
      visible: true,
      trackFeatures: "complex",
      featureThreshold: 1000,
      mouseover_callback: 'islandPopup',
      mouseout_callback: 'islandPopupClear',
      linear_mouseclick: 'linearPopup',
      showLabels: true,
      showTooltip: true,
      linear_mouseclick: 'linearClick',
      items: [
         {id: 1, start:1200, end:2450, name:"island0", strand: 1},
         {id: 2, start:3600,end:5700, name:"island1", strand: 1},
         {id: 3, start:6750,end:9800, name:"cheeky gene", strand: 1, extraclass: "outermembrane"},
      ]
   },
   {
      trackName: "track2",
      trackType: "track",
      visible: true,
      trackFeatures: "complex",
      featureThreshold: 1000,
      mouseover_callback: 'islandPopup',
      mouseout_callback: 'islandPopupClear',
      linear_mouseclick: 'linearPopup',
      showLabels: true,
      showTooltip: true,
      linear_mouseclick: 'linearClick',
      items: [
         {id: 3, start:4750,end:8800, name:"another cheeky gene", strand: 1},
      ]
   },
   { trackName: "covplot",
      trackType: "plot",
      visible: true,
      plot_min: 0.0,
      plot_max: 0.8,
      plot_mean: 0.5,
      bp_per_element: 1000,
      linear_plot_width: 50,
      linear_plot_height: 60,
      items: [0.0,0.0,0.0,0.0,0.0,0.6,0.8,0.5,0.6,0.6,0.0]
   },
];