Any way to remove outages from statistics?

Hello Chris,
thank you for your contact.

Data recorded by PRTG can't be modified, the only way to remove these 3 hours of data would be to remove the whole day worth of historic data and perform a Cache Recalculation. This will create a "gap" in your historic data for that day and affect all sensors coverage, but will undo any downtime caused by that outage.

Steps:

1. On the Core Server's filesystem, head to C:\ProgramData\Paessler\PRTG Network Monitor\Monitoring Database\, look for the folder for the date you want to remove (Format YYYYMMDD), for instance if the issue occurred yesterday, remove (or move to another location otuside PRTG's data folder) the folder "20160621".
2. Within PRTG's web-interface, head to Setup > Administrative Tools and issue a Recalculate PRTG Graph Data Cache.

Note1: that the time the Core Server will take to re-calculate the historic data cache can range from minutes to several hours depending on the amount of data stored on the server. the Live, 2, 30 and 365 days tabs of sensors will show incomplete or no values until the recalculation is completed.

Note2: If you wish to remove data for the current day, stop the Core Server Service before doing so as otherwise those files may be "open" by the core server.

Best Regards,
Luciano Lingnau [Paessler Support]