It's not clear to me how you defined "happiness" on the vertical axis. Can you elaborate more on that?

Hi orijing, I cover this in more detail elsewhere but the happiness is found through the ANEW dataset. This dataset scores a few thousand English words with sentiment values from 1 (very unhappy) to 9 (very happy). I find the happiness value for each word in a tweet and then average that value by the amount of words in a tweet to get the happiness value for each tweet.

How do you get access to this type of data?

The current version of ANEW can only be accessed for academic research. I do know that an older version is floating around in the open though, which they attached to a research paper.