input: 
  from typing import List, Dict
  import matplotlib.pyplot as plt
  
  def visualize_twitter_data(tweets: List[Dict]) -> None:
      """Visually summarize the Twitter data by plotting the number of tweets per author, the number of likes per tweet, and the number of retweets per tweet.
      >>> visualize_twitter_data([
      ...     {'author': 'User1', 'likes': 10, 'retweets': 5},
      ...     {'author': 'User2', 'likes': 20, 'retweets': 10},
      ...     {'author': 'User1', 'likes': 15, 'retweets': 7}
      ... ])
      """
      # Extract authors, num_tweets, num_likes, num_retweets
      authors = []
      num_tweets = []
      num_likes = []
      num_retweets = []
      
      for tweet in tweets:
          if tweet['author'] not in authors:
              authors.append(tweet['author'])
              num_tweets.append(1)
              num_likes.append(tweet['likes'])
              num_retweets.append(tweet['retweets'])
          else:
              index = authors.index(tweet['author'])
              num_tweets[index] += 1
              num_likes[index] += tweet['likes']
              num_retweets[index] += tweet['retweets']
      
      # Plot the tweet authors
      plt.figure(1)
      plt.xlabel('Tweet Authors')
      plt.ylabel('Number of tweets')
      plt.title('Tweet Author Summary')
      plt.bar(authors, num_tweets)
      plt.show()
      
      # Plot the number of likes
      plt.figure(2)
      plt.xlabel('Tweets')
      plt.ylabel('Number of Likes')
      plt.title('Number of Likes Summary')
      plt.bar(authors, num_likes)
      plt.show()
      
      # Plot the number of retweets
      plt.figure(3)
      plt.xlabel('Tweets')
      plt.ylabel('Number of Retweets')
      plt.title('Number of Retweets Summary')
      plt.bar(authors, num_retweets)
      plt.show()
  
output:     # Extract authors, num_tweets, num_likes, num_retweets
      authors = []
      num_tweets = []
      num_likes = []
      num_retweets = []
      
      for tweet in tweets:
          if tweet['author'] not in authors:
              authors.append(tweet['author'])
              num_tweets.append(1)
              num_likes.append(tweet['likes'])
              num_retweets.append(tweet['retweets'])
          else:
              index = authors.index(tweet['author'])
              num_tweets[index] += 1
              num_likes[index] += tweet['likes']
              num_retweets[index] += tweet['retweets']
      
      # Plot the tweet authors
      plt.figure(1)
      plt.xlabel('Tweet Authors')
      plt.ylabel('Number of tweets')
      plt.title('Tweet Author Summary')
      plt.bar(authors, num_tweets)
      plt.show()
      
      # Plot the number of likes
      plt.figure(2)
      plt.xlabel('Tweets')
      plt.ylabel('Number of Likes')
      plt.title('Number of Likes Summary')
      plt.bar(authors, num_likes)
      plt.show()
      
      # Plot the number of retweets
      plt.figure(3)
      plt.xlabel('Tweets')
      plt.ylabel('Number of Retweets')
      plt.title('Number of Retweets Summary')
      plt.bar(authors, num_retweets)
      plt.show()