Outputs per day is a better indicator of overall economic activity on the Bitcoin blockchain than transactions per day. Since one transaction can include multiple outputs, the number of total outputs is more important than the number of transactions themselves. Large players in the Bitcoin space use batching--the process of including multiple outputs in a given transaction--to reduce their overall transaction fees. Therefore, looking at only transactions misses an important part of the picture. So, we've created outputs.today to track Bitcoin outputs.