def write_multi_lvl_str_ref(formula, data)
      return if data.empty?

      @writer.tag_elements('c:multiLvlStrRef') do
        # Write the c:f element.
        write_series_formula(formula)

        @writer.tag_elements('c:multiLvlStrCache') do

          # Write the c:ptCount element.
          write_pt_count(data.last.size)

          # Write the data arrays in reverse order.
          data.reverse.each do |arr|
            @writer.tag_elements('c:lvl') do
              # Write the c:pt element.
              arr.each_with_index {|a, i| write_pt(i, a)}
            end
          end
        end
      end
    end
def write_multi_lvl_str_ref(formula, data)
      return if data.empty?

      @writer.tag_elements('c:multiLvlStrRef') do
        # Write the c:f element.
        write_series_formula(formula)

        @writer.tag_elements('c:multiLvlStrCache') do

          # Write the c:ptCount element.
          write_pt_count(data.last.size)

          # Write the data arrays in reverse order.
          data.reverse.each do |arr|
            @writer.tag_elements('c:lvl') do
              # Write the c:pt element.
              arr.each_with_index {|a, i| write_pt(i, a)}
            end
          end
        end
      end
    end
Write the <c:multiLvLStrRef> element.