I have managed to create liner trendlines using the following expression:
LINEST_M (TOTAL aggr(Sum(Refs),Month),Month)*Month+LINEST_B (TOTAL aggr(Sum(Refs),Month),Month)
However, I am interested in adding in seasonality to my data - for example I know that my data will drop off in December when there is less activity. Creating this in excel I would calculate the average monthly variance against the linear trendline and factor this against the forecasted trended data. The problem is that as my calculated dimension is Month this does not appear to work.
There is a field called 'M' which is the Month number - I assume though some kind of wizardry that this could be used to aggregate the data, but unfortunately it is currently beyond my ability.
Many thanks for any thoughts or workarounds.