# File lib/gruff/base.rb, line 900
    def make_stacked
      stacked_values = Array.new(@column_count, 0)
      @data.each do |value_set|
        value_set[1].each_with_index do |value, index|
          stacked_values[index] += value
        end
        value_set[1] = stacked_values.dup
      end
    end