Sometimes when we collect data, our systems will not update every 24-hr period for various reasons (e.g., server load, API prioritization, etc). Sometimes that happens over a few days, and you may see more or less a flat line with a repeated statistic.
In these cases, we have automatically taken the next updated day, and interpolated the change across the gap. This is why you see the same follower count across a social media platform chart, for example: this is the averaged change per day, across the gap of time.
In Spotify Popularity Index charts, these are usually direct from Spotify, as these are normalized ratings that sometimes do not change daily, which you can read more about here.