Hello Bordes remi
For the filter question - you can use the Last() function to do this.
Create a calculated field with this: IF last()=0 then 'show' else 'hide' end
Then set the field on the filter shelf and set "show" to be selected. The default for this calculation will be 'Table Across" so you do not need to modify the Table Calculation settings.
I believe the second portion of the question is to average the last 8 weeks BEFORE the current week you will need to do a dateadd() function to calculate out the start date for your filter and then compare to the current date. There are some great examples of this here - Base Relative Date Filter on Latest Date in Data and Current vs Previous Period to Date Comparison
I hope this helps