code: function Pivoter (options) {
      this.options = extend ({}, Pivoter.DEFAULT_OPTIONS, options);
      if (!this.options.x_field) throw new Error('No x_field config');
      if (!this.options.y_field) throw new Error('No y_field config');
      if (!this.options.value_field) throw new Error('No value_field config');
      if (!this.options.x_fields_list) throw new Error('No x_fields_list config');
    }
docs: /* options:
  x_field : this field is used for x axis ...
  y_field : this field is used for y axis ...
  value_field : this field is used for value ...
  x_fields_list : list of fields accross x axis ...
  init_empty_rows : should empty rows be inited,
  to_y : converting function to y ...
  from_y : convert back y value to original value ...
  pivot_init_value : what should we put as initial value ...