Did you mean Linear Regression?.
If you want for forcast by Linear Regression:
Go to Expression tab for the expression you want to forecast. (only works for your line chart)
Select Linear in Trend Lines.
Go to Axis tab, select the expression and include the no.of time buckets you want to forecast.
You shall find them in the attachement.
Custom regression using LR terms can be found in LINEST_* functions. Refer help manual
Hope this helps,
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