Javascript d3 js字母汇总组

下面是一个json数据，用于在图形中绘制多个矩形

data.push(["Shanghai","Los Angeles","33333333.33"]);
data.push(["Los Angeles","Shanghai","222222.33"]);

data.push(["Los Angeles","Shanghai","22.33"]);
data.push(["chennai","Bombay","33333333.33"]);

data.push(["Canada","Japan","33333333.33"]);
data.push(["Japan","Canada","222222.33"]);

data.push(["Bombay","Canada","222222222.33"]);
data.push(["Bombay","Belgaum","222222.33"]);
data.push(["Bombay","Delhi","222222.33"]);

如何基于第一个字符数据（如group by B、C等）对上述数据进行分组您可以使用

d3.nest（）

看

var d = d3.nest()
  .key(function(d) { 
      return d[0].charAt(0); // 1st char of the 1st item of the array
   })
  .entries(data);