The Total Mail Storage Over Time report shows you the total MB and items stored per day in your Office 365 Environment. By default, the chart displays the previous month's worth of data as a bar graph.
Modifying the Chart
This report can also be displayed as a line graph, and you can hide or show the different series items. You can find more information about modifying the chart at this support article.
Changing the Date Period of the Report
When you first view this report it will show you the last 30 days of information. Use the Date Selection tool to display a different date range.
The Data Table
The data table lists each date, the total storage size in MB, and the total number of items. You can add, remove or re-order these columns to display the information in a custom way.
As with all of the reports, this information can be customized, filtered and scheduled.