input: Write a function to visually summarize Twitter data. The input is a stream of tweets with data fields like tweet author, date of tweet, text, number of likes, and number of retweets. The function should generate three bar charts: one summarizing the number of tweets per author, one summarizing the number of likes per tweet, and one summarizing the number of retweets per tweet. If the input data is empty, the function should return a message indicating no data is available.
  assert visualize_twitter_data([{'author': 'user1', 'date': '2023-01-01', 'text': 'Hello', 'likes': 10, 'retweets': 5}, {'author': 'user2', 'date': '2023-01-02', 'text': 'World', 'likes': 5, 'retweets': 3}, {'author': 'user1', 'date': '2023-01-03', 'text': 'Python', 'likes': 7, 'retweets': 2}])==None
  assert visualize_twitter_data([])=='No data available'
output: def visualize_twitter_data(tweets):
      if not tweets:
          return 'No data available'
      
      import matplotlib.pyplot as plt
      
      authors = [tweet['author'] for tweet in tweets]
      likes = [tweet['likes'] for tweet in tweets]
      retweets = [tweet['retweets'] for tweet in tweets]
      
      author_counts = {author: authors.count(author) for author in set(authors)}
      
      plt.figure(figsize=(15, 5))
      
      plt.subplot(1, 3, 1)
      plt.bar(author_counts.keys(), author_counts.values())
      plt.title('Tweets per Author')
      
      plt.subplot(1, 3, 2)
      plt.bar(range(len(likes)), likes)
      plt.title('Likes per Tweet')
      
      plt.subplot(1, 3, 3)
      plt.bar(range(len(retweets)), retweets)
      plt.title('Retweets per Tweet')
      
      plt.show()