input: 
  def summarize_twitter_data(twitter_data):
      """
      Visually summarize the Twitter data by plotting the number of tweets by author, 
      the number of likes per tweet, and the number of retweets per tweet.
      
      >>> twitter_data = [
      ...     {'author': 'User1', 'date': '2023-10-01', 'text': 'Sample tweet 1', 'likes': 10, 'retweets': 5},
      ...     {'author': 'User2', 'date': '2023-10-02', 'text': 'Sample tweet 2', 'likes': 15, 'retweets': 3},
      ...     {'author': 'User1', 'date': '2023-10-03', 'text': 'Sample tweet 3', 'likes': 8, 'retweets': 2}
      ... ]
      >>> summarize_twitter_data(twitter_data)
      """
      from collections import Counter
      import matplotlib.pyplot as plt
  
      # Extract relevant data
      authors = [tweet['author'] for tweet in twitter_data]
      num_likes = [tweet['likes'] for tweet in twitter_data]
      num_retweets = [tweet['retweets'] for tweet in twitter_data]
      tweets = range(len(twitter_data))
      authors_count = Counter(authors)
  
      # Plot the tweet authors
      plt.xlabel('Tweet Authors')
      plt.ylabel('Number of tweets')
      plt.title('Tweet Author Summary')
      plt.bar(authors_count.keys(), authors_count.values())
      plt.show()
  
      # Plot the number of likes
      plt.xlabel('Tweets')
      plt.ylabel('Number of Likes')
      plt.title('Number of Likes Summary')
      plt.bar(tweets, num_likes)
      plt.show()
  
      # Plot the number of retweets
      plt.xlabel('Tweets')
      plt.ylabel('Number of Retweets')
      plt.title('Number of Retweets Summary')
      plt.bar(tweets, num_retweets)
      plt.show()
  
output:     # Plot the number of retweets
      plt.xlabel('Tweets')
      plt.ylabel('Number of Retweets')
      plt.title('Number of Retweets Summary')
      plt.bar(tweets, num_retweets)
      plt.show()