Some of the more common reasons for seeing large, negative spikes amongst a project's performance or generation data are when an on-site production meter, from which PowerHub is acquiring data, has stopped working, gone off line or has been reset to zero.
If the gap created by one of these causes has been corrected on your Data Acquisition Service (DAS) provider's end, the discrepancy can be corrected in PowerHub using the 
Manual Import.
Please contact your Organization's Administrator if you need further details on how to access the Manual Import.