diff --git a/lib/git_stats/git_data/blob.rb b/lib/git_stats/git_data/blob.rb
index d4b717290..3c07c1dcf 100644
--- a/lib/git_stats/git_data/blob.rb
+++ b/lib/git_stats/git_data/blob.rb
@@ -15,6 +15,10 @@ module GitStats
         @content ||= Command.new(repo, "git cat-file blob #{self.hash}").run
       end
 
+      def extension
+        @ext ||= File.extname(filename)
+      end
+
       def to_s
         "#{self.class} #@hash #@filename"
       end
diff --git a/lib/git_stats/git_data/commit.rb b/lib/git_stats/git_data/commit.rb
index 5b4540efb..5ade7ad51 100644
--- a/lib/git_stats/git_data/commit.rb
+++ b/lib/git_stats/git_data/commit.rb
@@ -15,6 +15,14 @@ module GitStats
         end
       end
 
+      def files_by_extension
+        @files_by_extension ||= files.inject({}) { |acc, f| acc[f.extension] ||= []; acc[f.extension] << f; acc }
+      end
+
+      def lines_by_extension
+        @lines_by_extension ||= Hash[files_by_extension.map { |ext, files| [ext, files.map(&:lines_count).sum] }]
+      end
+
       def files_count
         @files_count ||= Command.new(repo, "git ls-tree -r --name-only #{self.hash} | wc -l").run.to_i
       end
diff --git a/lib/git_stats/hash_initializable.rb b/lib/git_stats/hash_initializable.rb
index 5775c229a..864e44332 100644
--- a/lib/git_stats/hash_initializable.rb
+++ b/lib/git_stats/hash_initializable.rb
@@ -1,5 +1,6 @@
 module HashInitializable
   def initialize(params = {})
+    raise "pass a Hash to initialize #{self.class}" unless params.is_a? Hash
     params.each { |k, v| instance_variable_set("@#{k}", v) }
   end
 end
\ No newline at end of file
diff --git a/lib/git_stats/stats_view/view_data.rb b/lib/git_stats/stats_view/view_data.rb
index 7ebbe0eba..ae7a943a3 100644
--- a/lib/git_stats/stats_view/view_data.rb
+++ b/lib/git_stats/stats_view/view_data.rb
@@ -42,6 +42,26 @@ module GitStats
         end
       end
 
+      def files_by_extension
+        @files_by_extension ||= LazyHighCharts::HighChart.new('graph') do |f|
+          f.chart(type: "column")
+          f.title(text: "files by extension")
+          f.xAxis(categories: repo.commits.last.files_by_extension.keys)
+          f.yAxis(min: 0, title: {text: 'Commits'})
+          f.series(name: "commits", data: repo.commits.last.files_by_extension.values.map(&:count))
+        end
+      end
+
+      def lines_by_extension
+        @lines_by_extension ||= LazyHighCharts::HighChart.new('graph') do |f|
+          f.chart(type: "column")
+          f.title(text: "files by extension")
+          f.xAxis(categories: repo.commits.last.lines_by_extension.keys)
+          f.yAxis(min: 0, title: {text: 'Commits'})
+          f.series(name: "commits", data: repo.commits.last.lines_by_extension.values)
+        end
+      end
+
       def files_by_date
         @files_by_date ||= LazyHighCharts::HighChart.new('graph') do |f|
           f.title(text: "Files")
diff --git a/spec/author_spec.rb b/spec/author_spec.rb
index 168cafdfb..d57234eee 100644
--- a/spec/author_spec.rb
+++ b/spec/author_spec.rb
@@ -1,7 +1,7 @@
 require 'spec_helper'
 
 describe GitStats::GitData::Author do
-  let(:repo) { GitStats::GitData::Repo.new("repo_path") }
+  let(:repo) { GitStats::GitData::Repo.new(path: "repo_path") }
   let(:author) { GitStats::GitData::Author.new(repo: repo, name: "author1", email: "author1@gmail.com") }
   let(:other_author) { GitStats::GitData::Author.new(repo: repo, name: "author2", email: "author2@gmail.com") }
   let(:my_commits) { 10.times.map { |i| double("my_commit #{i}", :author => author) } }
diff --git a/spec/generator_spec.rb b/spec/generator_spec.rb
index b14d227d9..9c34dfb61 100644
--- a/spec/generator_spec.rb
+++ b/spec/generator_spec.rb
@@ -19,7 +19,7 @@ describe GitStats::Generator do
 
   it 'should render all templates with view data for this repo' do
     repo = double('repo')
-    GitStats::GitData::Repo.should_receive(:new).with(repo_path).and_return(repo)
+    GitStats::GitData::Repo.should_receive(:new).with(path: repo_path, git_command_observer: nil).and_return(repo)
 
     view_data = double('view_data')
     GitStats::StatsView::ViewData.should_receive(:new).with(repo).and_return(view_data)
diff --git a/spec/repo_spec.rb b/spec/repo_spec.rb
index 43755daed..d6c8b4766 100644
--- a/spec/repo_spec.rb
+++ b/spec/repo_spec.rb
@@ -1,7 +1,7 @@
 require 'spec_helper'
 
 describe GitStats::GitData::Repo do
-  let(:repo) { GitStats::GitData::Repo.new("repo_path") }
+  let(:repo) { GitStats::GitData::Repo.new(path: "repo_path") }
 
   describe 'git output parsing' do
     before {
diff --git a/templates/files.haml b/templates/files.haml
index 52505d908..bf5661ae9 100644
--- a/templates/files.haml
+++ b/templates/files.haml
@@ -1 +1,2 @@
 = high_chart("files_by_date", files_by_date)
+= high_chart("files_by_extension", files_by_extension)
\ No newline at end of file
diff --git a/templates/lines.haml b/templates/lines.haml
index fa40ba037..1392e6835 100644
--- a/templates/lines.haml
+++ b/templates/lines.haml
@@ -1 +1,2 @@
-= high_chart("lines_by_date", lines_by_date)
\ No newline at end of file
+= high_chart("lines_by_date", lines_by_date)
+= high_chart("lines_by_extension", lines_by_extension)
\ No newline at end of file