When you create a new custom report, the data for past dates will only be available for the period configured in your configuration file. Please note the confgiration file is not available to Cloud customers. For more details please see our guide here: How do I see historical data for a new Custom Report?

For gathering data for even further back than that configured in your config file you can:
1. Invalidate the historical data, and
2. then re-process the reports for all your websites. If you have setup auto-archiving then the command to re-process the reports will automatically run within the next hour or so. Alternatively you can manually re-process the reports. Depending on the amount of data in your Matomo (Piwik), this command may take a while to run and you may want to adjust the amount of days (and seconds) you want to reprocess.

Cloud customers will by default have the previous 30 days of data prior to the date of the report being created available.

Previous FAQ: Can I pivot Custom Reports?