def sort(label, ascending=true)
      i = @labels.index(label)
      raise "No column named" + label.to_s if i.nil?
      order = @data[i].sorted_indices
      order.reverse! unless ascending
      self.sort_by.with_index{|val, i| order.index(i)}
    end
def sort(label, ascending=true)
      i = @labels.index(label)
      raise "No column named" + label.to_s if i.nil?
      order = @data[i].sorted_indices
      order.reverse! unless ascending
      self.sort_by.with_index{|val, i| order.index(i)}
    end
Sort by label
 @param [Symbol] label column name to sort by
 @param [Bool] ascending default true