docs: function widen(widening, width) {
    widening = widening || 0;
  
    return function(d, i, array) {
      if(array.length > 2) throw "array.length > 2 unsupported. array.length = " + array.length;
      width = width || (array[array.length-1] - array[0]);
      return d + (i*2-1)*width*widening;
    };
  }