Each dot on these graphs indicates a retweet of a specific tweet (distinguished by colors). The size of the dot is proportional to the number of followers that the retweeter has (a proxy for the size of the audience exposed to the tweet). The Y-value of the dot is the cumulative sum of the number of followers for each retweeter. The Y-axis then represents the potential size of the audience that has been exposed to the tweet. Note these cumulative sums do may include duplicate followers.