You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: parser.rb
    path of file: ./repos/YouPlot/lib/youplot
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

require 'optparse'
require_relative 'options'
    
    def sub_parser_add_fmt_yx
      sub_parser.on_head('--fmt STR', String,
                         'xy : header is like x, y...',
                         'yx : header is like y, x...') do |v|
        options[:fmt] = v
      end
    end