Refactor: replace "render :json = graph.to_json" to view template(show.json.erb).

Because model shouldn't know about view logic.

app/views/graph/show.json.erb

@@ -0,0 +1,26 @@
+<% self.formats = ["html"] %>
+
+<%= raw(
+  {
+    days: @graph.days.compact.map { |d| [d.day, d.strftime("%b")] },
+    commits: @graph.commits.map do |c|
+      {
+        parents: c.parents.collect do |p|
+          [p.id,0,0]
+        end,
+        author: {
+          name: c.author.name,
+          email: c.author.email,
+          icon: gravatar_icon(c.author.email, 20)
+        },
+        time: c.time,
+        space: c.spaces.first,
+        parent_spaces: c.parent_spaces,
+        refs: join_with_space(c.refs),
+        id: c.sha,
+        date: c.date,
+        message: c.message,
+      }
+    end
+  }.to_json
+) %>