However, at certain point, POS_AMT values for few weeks are Blanks, with just FORECAST_AMT column being filed.
What I would want to achieve is -> For WEEK_END_DATE values that have FORECAST_AMT, but doesn't have POS_AMT use FORECAST_AMT insead of POS_AMT in cumulative calculation.
The other way would be to calculate cumulative POS_AMT for only those weeks that have such values and later on calculate another measure with FORECAST_AMT, but only for those weeks that doesn't have POS_AMT.
We can’t color them separately due to we accumulate the total. If the first month uses FROCAST_AMT, we can say the last month uses it too. If we want to find out the current month using it or not, we can add a calculated column, and then add it to legend. This doesn’t work with area chart. You can have a look at the image below. The black column means this column uses FROCAST_AMT. Is this what you want?
IF ( MIN ( ARD_POS_FRCST_WKLY[POS_AMT] ) = 0, 1, 0 ),