def write_tx_formula(title, data_id) # :nodoc:
      data = @formula_data[data_id] if data_id

      @writer.tag_elements('c:tx') { write_str_ref(title, data, 'str') }
    end
def write_tx_formula(title, data_id) # :nodoc:
      data = @formula_data[data_id] if data_id

      @writer.tag_elements('c:tx') { write_str_ref(title, data, 'str') }
    end
Write the <c:tx> element.