How to compare to a previous period within a report
Open the report design editor, and click the date pulldown at the top of the screen.
Enable the "compare to previous" option, choosing your desired comparison period from the drop down.
Previous Period: If you choose "Last 7 days", you will see a comparison to the 7 days prior to the current 7 day period. If you choose "This month", you'll see a comparison to last month. If you choose a custom date range that spans 47 days, then the comparison will be to the previous 47 day period.
Previous Year: This is based on calendar year. If you choose "Last 7 days", then your comparison will be to the exact same calendar dates of the previous calendar year. If you choose "This month", you'll see a comparison to the same month for the previous calendar year. And so on.
Note: The "Compare to Previous" comparisons are generally displayed as either a percentage increase/decrease or an up/down number. The specific metrics used to calculate these numbers will vary based on the module and the metric.
Working with Line Charts
When using "Compare to Previous", your line charts will display a secondary "previous period" line on them. Here's how it looks: