trend line gradient value

Hi Gareth_Hunt ,

Are you try to calculating sales change percentage over time? It's better that you can create a calendar table in your data model, and use the previousmonth function(aggregation of month). For more details, please refer to https://www.mssqltips.com/sqlservertip/6424/calculate-percentage-growth-over-time-with-power-bi/

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Best Regards,

Dedmon Dai