analytics.html.erb

<% @categories.each do |category, category_count| %>
  <tr class="<%= category %> value" data-count="<%= category_count %>">
    <td><%= category_count %></td><td><%= category_count * 100 / @violations_to_check %>%</td><td><%= category_count %></td><td><i class="icon-arrow-up"></i></td>
  </tr>
<% end %>

code_reviews_controller.rb

def analytics
  @code_review = CodeReview.find(params[:id])
  @categories = @code_review.violations.group('category').count
  @violations_to_check = @code_review.violations_to_check.size # just to be on the safe side if this does trigger multiple queries in the loop in the view
end