Google Maps V3 API - Center on a category pulling from xml file

I'm working with this example from Geocodezip

The markers on the map change depending on which category is selected. I would like them to automatically center on the category of markers but I'm having difficulty getting it to work.

The issue I'm having with all the fitbounds examples I've found on here and elsewhere is that they require the markers to be defined in the Javascript whereas mine are being imported from an XML file. I'm not sure what to try next.

The script:

<script type="text/javascript">
    //<![CDATA[
      // this variable will collect the html which will eventually be placed in the side_bar 
      var side_bar_html = ""; 

      var gmarkers = [];
      var gicons = [];
      var map = null;

var infowindow = new google.maps.InfoWindow(
  { 
    size: new google.maps.Size(150,50)
  });


gicons["red"] = new google.maps.MarkerImage("mapIcons/marker_red.png",
      // This marker is 20 pixels wide by 34 pixels tall.
      new google.maps.Size(20, 34),
      // The origin for this image is 0,0.
      new google.maps.Point(0,0),
      // The anchor for this image is at 9,34.
      new google.maps.Point(9, 34));
  // Marker sizes are expressed as a Size of X,Y
  // where the origin of the image (0,0) is located
  // in the top left of the image.

  // Origins, anchor positions and coordinates of the marker
  // increase in the X direction to the right and in
  // the Y direction down.

  var iconImage = new google.maps.MarkerImage('mapIcons/marker_red.png',
      // This marker is 20 pixels wide by 34 pixels tall.
      new google.maps.Size(20, 34),
      // The origin for this image is 0,0.
      new google.maps.Point(0,0),
      // The anchor for this image is at 9,34.
      new google.maps.Point(9, 34));
  var iconShadow = new google.maps.MarkerImage('http://www.google.com/mapfiles/shadow50.png',
      // The shadow image is larger in the horizontal dimension
      // while the position and offset are the same as for the main image.
      new google.maps.Size(37, 34),
      new google.maps.Point(0,0),
      new google.maps.Point(9, 34));
      // Shapes define the clickable region of the icon.
      // The type defines an HTML &lt;area&gt; element 'poly' which
      // traces out a polygon as a series of X,Y points. The final
      // coordinate closes the poly by connecting to the first
      // coordinate.
  var iconShape = {
      coord: [9,0,6,1,4,2,2,4,0,8,0,12,1,14,2,16,5,19,7,23,8,26,9,30,9,34,11,34,11,30,12,26,13,24,14,21,16,18,18,16,20,12,20,8,18,4,16,2,15,1,13,0],
      type: 'poly'
  };

function getMarkerImage(iconColor) {
   if ((typeof(iconColor)=="undefined") || (iconColor==null)) { 
      iconColor = "red"; 
   }
   if (!gicons[iconColor]) {
      gicons[iconColor] = new google.maps.MarkerImage("mapIcons/marker_"+ iconColor +".png",
      // This marker is 20 pixels wide by 34 pixels tall.
      new google.maps.Size(20, 34),
      // The origin for this image is 0,0.
      new google.maps.Point(0,0),
      // The anchor for this image is at 6,20.
      new google.maps.Point(9, 34));
   } 
   return gicons[iconColor];

}

function category2color(category) {
   var color = "red";
   switch(category) {
     case "theatre": color = "blue";
                break;
     case "golf":    color = "green";
                break;
     case "info":    color = "yellow";
                break;
     default:   color = "red";
                break;
   }
   return color;
}

      gicons["theatre"] = getMarkerImage(category2color("theatre"));
      gicons["golf"] = getMarkerImage(category2color("golf"));
      gicons["info"] = getMarkerImage(category2color("info"));

      // A function to create the marker and set up the event window
function createMarker(latlng,name,html,category) {
    var contentString = html;
    var marker = new google.maps.Marker({
        position: latlng,
        icon: gicons[category],
        shadow: iconShadow,
        map: map,
        title: name,
        zIndex: Math.round(latlng.lat()*-100000)<<5
        });
        // === Store the category and name info as a marker properties ===
        marker.mycategory = category;                                 
        marker.myname = name;
        gmarkers.push(marker);

    google.maps.event.addListener(marker, 'click', function() {
        infowindow.setContent(contentString); 
        infowindow.open(map,marker);
        });
}

      // == shows all markers of a particular category, and ensures the checkbox is checked ==
      function show(category) {
        for (var i=0; i<gmarkers.length; i++) {
          if (gmarkers[i].mycategory == category) {
            gmarkers[i].setVisible(true);
          }
        }
        // == check the checkbox ==
        document.getElementById(category+"box").checked = true;
      }

      // == hides all markers of a particular category, and ensures the checkbox is cleared ==
      function hide(category) {
        for (var i=0; i<gmarkers.length; i++) {
          if (gmarkers[i].mycategory == category) {
            gmarkers[i].setVisible(false);
          }
        }
        // == clear the checkbox ==
        document.getElementById(category+"box").checked = false;
        // == close the info window, in case its open on a marker that we just hid
        infowindow.close();
      }

      // == a checkbox has been clicked ==
      function boxclick(box,category) {
        if (box.checked) {
          show(category);
        } else {
          hide(category);
        }
        // == rebuild the side bar
        makeSidebar();
      }

      function myclick(i) {
        google.maps.event.trigger(gmarkers[i],"click");
      }


      // == rebuilds the sidebar to match the markers currently displayed ==
      function makeSidebar() {
        var html = "";
        for (var i=0; i<gmarkers.length; i++) {
          if (gmarkers[i].getVisible()) {
            html += '<a href="javascript:myclick(' + i + ')">' + gmarkers[i].myname + '<\/a><br>';
          }
        }
        document.getElementById("side_bar").innerHTML = html;
      }

  function initialize() {
    var myOptions = {
      zoom: 11,
      center: new google.maps.LatLng(53.8363,-3.0377),
      mapTypeId: google.maps.MapTypeId.ROADMAP
    }
    map = new google.maps.Map(document.getElementById("map"), myOptions);


    google.maps.event.addListener(map, 'click', function() {
        infowindow.close();
        });



      // Read the data
      downloadUrl("categories.xml", function(doc) {
  var xml = xmlParse(doc);
  var markers = xml.documentElement.getElementsByTagName("marker");

        for (var i = 0; i < markers.length; i++) {
          // obtain the attribues of each marker
          var lat = parseFloat(markers[i].getAttribute("lat"));
          var lng = parseFloat(markers[i].getAttribute("lng"));
          var point = new google.maps.LatLng(lat,lng);
          var address = markers[i].getAttribute("address");
          var name = markers[i].getAttribute("name");
          var html = "<b>"+name+"<\/b><p>"+address;
          var category = markers[i].getAttribute("category");
          // create the marker
          var marker = createMarker(point,name,html,category);
        }

        // == show or hide the categories initially ==
        show("theatre");
        hide("golf");
        hide("info");
        // == create the initial sidebar ==
        makeSidebar();
      });
    }

    // This Javascript is based on code provided by the
    // Community Church Javascript Team
    // http://www.bisphamchurch.org.uk/   
    // http://econym.org.uk/gmap/
    // from the v2 tutorial page at:
    // http://econym.org.uk/gmap/example_categories.htm
    //]]>

Answers:

Answer

To zoom to the last "shown" category, add the markers to a google.maps.LatLngBounds object, then call map.fitBounds with that object as its argument.

// == shows all markers of a particular category, and ensures the checkbox is checked, zooms to the displayed markers of this category ==
function show(category) {
  var bounds = new google.maps.LatLngBounds();
  for (var i=0; i<gmarkers.length; i++) {
    if (gmarkers[i].mycategory == category) {
      gmarkers[i].setVisible(true);
      bounds.extend(gmarkers[i].getPosition());
    }
  }
  map.fitBounds(bounds);
  // == check the checkbox ==
  document.getElementById(category+"box").checked = true;
}

working example

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