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Maintain historical Data
Hello all, I have a transaction system that contains percentage completion and status of Projects. The thing is, when the percentage completion of a project is updated, the previous value is lost an...
You cannot. Power BI is a "read only" system so it doesn't have the ability to store data beyond a refresh.
You will either have to modify the source system (you said not an option), or put in an intermediate process in place to store a previous day snapshot of data, then have Power BI read in the source data and the snapshot, then do the comparison.
If you are using power BI and not just Excel, you can check out this video which will explain how to populate a history table using incremental refresh. If you can modify the Query a bit, this would do the trick!