谷歌铁轨上的图表

你好,我一直在读关于如何在rails上使用谷歌图表制作图表的文章,我已经安装了插件,但我不知道为什么会显示这个错误:

NameError in TankingLogsController#index

uninitialized constant TankingLogsController::GoogleChart

这是我的inde.html.erb

<div class="container">
 <%= image_tag @bar %>

  <h1>Listing Tankings</h1>
  <% if @tankinglog.count<1 %>
    <p>
    There are no tankings for this car. Do you want to <%= link_to 'create a new tanking', new_user_car_tanking_log_path(@user, @car)%>
    </p>
  <% else %>
    <table class="table table-condensed">
      <tr>
        <th>Cost</th>
        <th>Gallon</th>
        <th>Km</th>
        <th>Date</th>
        <th>Gas Station's id</th>
        <th></th>
      </tr>
      <% @tankinglog.each do |tankinglog| %>
        <tr>
          <td><%= tankinglog.cost %></td>
          <td><%= tankinglog.gallon %></td>
          <td><%= tankinglog.km %></td>
          <td><%= tankinglog.date %></td>
          <td><%= tankinglog.gas_station_id %></td>
        </tr>
      <% end %>
    </table>
    <br />
    <%= link_to 'New tanking', new_user_car_tanking_log_path(@user, @car), :class => "btn btn-primary" %>
  <% end %>
  <br />
  <br />
  <%= link_to 'back', user_cars_path(current_user), :class => "btn btn-primary" %>
  </div> 

这是我的控制器

class TankingLogsController < ApplicationController

....


  def index
    @user = User.find(params[:user_id]) 
    @car = @user.cars.find(params[:car_id])    
    @tankinglog = @car.tanking_logs.all
    @bar = GoogleChart.pie(['1997',10],['1998',20],['1999',40],['2000',30]).to_url
  end
end

我的宝石档案

source 'https://rubygems.org'

gem 'rails', '3.2.6'
gem 'pg', '0.13.2'
gem 'devise', '2.0.4'
gem 'haml-rails'
gem 'twitter-bootstrap-rails', '2.0.6'
gem 'sass-rails',   '~> 3.2.3'
gem "rspec", "~> 2.10.0"
gem 'simplecov', :require => false, :group => :test
gem 'gchart'