In the top graph, all the metrics are reported based on each month. For instance in Jan 2016, revenue was about $4M, with daily average sales of $130k. The smallest day in sales for that month was $57,455 which also happened to be the smallest sales day of the year. On the graph to the right, we have a daily avg report where it is possible to drill up or down to view the daily average by day, month, quarter or year.
The green and red dashed lines in the "Daily Avg by Date" graph are the 80th and 20th percentiles respectively. You can compute this using the analytics feature. While this is a handy built-in feature, it is also useful to have a sense of how this is computed. Learn more about percentiles here.