Omit the toggle button if there are no disabled agents

app/views/diagrams/show.html.erb

@@ -10,10 +10,12 @@
       </div>
       <div class="btn-group">
         <%= link_to icon_tag('glyphicon-chevron-left') + ' Back'.html_safe, (@scenario ? scenario_path(@scenario) : agents_path), class: "btn btn-default" %>
-        <% if params[:include_disabled] %>
-          <%= link_to @scenario ? scenario_diagram_path(@scenario) : diagram_path, class: 'btn btn-default' do %><%= icon_tag('glyphicon-eye-close') %> Hide <%= pluralize(@disabled_agents.count, 'disabled Agent') %><% end %>
-        <% else %>
-          <%= link_to @scenario ? scenario_diagram_path(@scenario, include_disabled: true) : diagram_path(include_disabled: true), class: 'btn btn-default' do %><%= icon_tag('glyphicon-eye-open') %> Show <%= pluralize(@disabled_agents.count, 'disabled Agent') %><% end %>
+        <% if (num_disabled = @disabled_agents.count).nonzero? -%>
+          <% if params[:include_disabled] %>
+            <%= link_to @scenario ? scenario_diagram_path(@scenario) : diagram_path, class: 'btn btn-default' do %><%= icon_tag('glyphicon-eye-close') %> Hide <%= pluralize(num_disabled, 'disabled Agent') %><% end %>
+          <% else %>
+            <%= link_to @scenario ? scenario_diagram_path(@scenario, include_disabled: true) : diagram_path(include_disabled: true), class: 'btn btn-default' do %><%= icon_tag('glyphicon-eye-open') %> Show <%= pluralize(num_disabled, 'disabled Agent') %><% end %>
+          <% end %>
         <% end %>
       </div>