在Rails中使用Google Visualizer时,第一列和第二列必须是实体和时间,为什么?

我正在使用一个Ruby包装器,它似乎适合使用Google可视化工具:

https://github.com/jeremyolliver/gvis

但是,我还是会遇到一个特别的错误:

First and second columns must be entity and time 

然而,据我所知,我传递的数据是实体和时间:

  <% chart_data = [
 ["Apples", Date.new(1998,1,1), 1000,300,'East'],
 ["Oranges", Date.new(1998,1,1), 950,200,'West'],
 ["Bananas", Date.new(1998,1,1), 300,250,'West'],
 ["Apples", Date.new(1998,2,1), 1200,400,'East'],
 ["Oranges", Date.new(1998,2,1), 950,150,'West'],
 ["Bananas", Date.new(1998,2,1), 788,617,'West']
 ] %>

<% visualization "my_chart", "MotionChart", :width => 600, :height => 400, :html => {:class => "graph_chart"} do |chart| %>
<%# Add the columns that the graph will have
<% chart.string "Fruit" %>
<% chart.date "Date" %>
<% chart.number "Sales" %>
<% chart.number "Expenses" %>
<% chart.string "Location" %>

<% chart.add_rows(chart_data) %>

<%结束%>