It's quite tricky what you want to do and the solution depends on your exact requirements.You said you want one side of a dual axis to move with a filter and the other side to stay static. In this case you can duplicate the data source. Tableau automatically 'blends' the 2 data sources but the filter only filters the primary data source, not the 2nd - so the Average Sales in this example remains static.
See the workbook attached as I realise I haven't described how to do this too well. I'll attempt to write it up better with a step by step guide when I get a chance. Hopefully you can work it out from the attached.