[flot] returned canvas reuse

This gives us _massive_ improvement in memory use. Before that Chrome
could easily grab as much as 400M for heap, with this patch it seems
to stay way below 100M.

This basically reverts 6fe0258 back.
Which applied flot patch from here:
http://code.google.com/p/flot/issues/detail?id=269

Change-Id: Icf9bd8d828c1c36e4fdd0886bca44407148ef2b7
Reviewed-on: http://review.membase.org/3319
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Steve Yen <steve.yen@gmail.com>

deps/menelaus/priv/js/jquery.flot.js

@@ -540,10 +540,26 @@
                     c = window.G_vmlCanvasManager.initElement(c);
                 return c;
             }
-            
+
             canvasWidth = placeholder.width();
             canvasHeight = placeholder.height();
-            placeholder.html(""); // clear placeholder
+            var checkCanvas = placeholder.children('canvas');
+            if (checkCanvas.length &&
+                (checkCanvas.get(0).width == canvasWidth) &&
+                (checkCanvas.get(0).height == canvasHeight)){
+                canvas = checkCanvas.get(0);
+                ctx = canvas.getContext('2d');
+                overlay = checkCanvas.get(1);
+                octx = overlay.getContext('2d');
+
+                //do whatever manual clearing is necessary here
+                ctx.clearRect(0,0,canvasWidth,canvasHeight);
+                octx.clearRect(0,0,canvasWidth,canvasHeight);
+                unbindEvents();
+            } else {
+                placeholder.get(0).innerHTML = ''; //this helps with leaky memory possibly
+                //placeholder.html(""); // clear placeholder
+
             if (placeholder.css("position") == 'static')
                 placeholder.css("position", "relative"); // for positioning labels and overlay
 
@@ -552,7 +568,7 @@
 
             if ($.browser.msie) // excanvas hack
                 window.G_vmlCanvasManager.init_(document); // make sure everything is setup
-            
+
             // the canvas
             canvas = $(makeCanvas(canvasWidth, canvasHeight)).appendTo(placeholder).get(0);
             ctx = canvas.getContext("2d");
@@ -561,6 +577,7 @@
             overlay = $(makeCanvas(canvasWidth, canvasHeight)).css({ position: 'absolute', left: 0, top: 0 }).appendTo(placeholder).get(0);
             octx = overlay.getContext("2d");
             octx.stroke();
+            }
         }
 
         function bindEvents() {
@@ -578,6 +595,13 @@
             executeHooks(hooks.bindEvents, [eventHolder]);
         }
 
+        function unbindEvents() {
+            // we include the canvas in the event holder too, because IE 7
+            // sometimes has trouble with the stacking order
+            eventHolder = $([overlay, canvas]);
+            eventHolder.unbind();
+        }
+
         function setupGrid() {
             function setTransformationHelpers(axis, o) {
                 function identity(x) { return x; }