按时间段分组
PostgreSQL：
<div id="chart"></div><%= Seer::visualize( @user_counts, :as => :column_chart, :in_element =>'chart', :series => { :series_label => 'date', :data_method => 'count' }, :chart_options => { :height => 300, :width => 100 * @user_counts.size, :is_3_d => true, :legend => 'none', :colors => "[{color:'#990000', darker:'#660000'}]", :title => "New users in last 7 days", :title_x => 'date', :title_y => 'count' } ) -%>
