added clear canvas button back to topics view, including javascript code to back it up

app/assets/javascripts/Jit/find.js

@@ -164,9 +164,10 @@ function clearCanvasExceptRoot() {
   ids.forEach(function(id, index) {
     if (id != root.id) {
       Mconsole.graph.removeNode(id);
-      //OK I feel bad about this, but not too bad
-      //TODO: this leaves labels hidden on the map
+      //don't use disposeLabel or they'll never come back!
+      //maybe there's a better way that recreates the labels later??
       Mconsole.labels.hideLabel(id);
+      $('#topic_' + id + '_label').hide();
     }
   });
   fetchRelatives(root); //also runs Mconsole.plot()

app/assets/javascripts/Jit/graphsettings.js

@@ -507,6 +507,7 @@ function onPlaceLabelHandler(domElement, node) {
   style.top = (top-165) + 'px';
   style.display = ''; 
   var label = document.getElementById('topic_' + node.id + '_label');
+  $(label).show(); 
   w = label.offsetWidth;
   style = label.style;
   style.left = (-(w / 2 + 106)) + 'px';    

app/views/topics/show.html.erb

@@ -10,7 +10,7 @@
   # map recenters on a new topic, it's like you're on that topic's page.
   # Nice, but the URL and being unable to remove the root node still hamper that
   # experience.
-  # TODO: What URL accesses this view?
+  # URL: /topics/<topicid>
   #
   #%>
 
@@ -18,7 +18,7 @@
   <% if authenticated? %>
     <button onclick="saveToMap();">Save to Map</button>
   <% end %>
-  <!-- <button onclick='clearCanvasExceptRoot();'>Clear Canvas</button> -->
+  <button onclick='clearCanvasExceptRoot();'>Clear Canvas</button>
 </div>
 <div class="clearfloat"></div>