Creating Different Sort Order based on what level is Drilled Down On

I have a bar chart which shows the cases due in the next 7 days sorted by date.  

It has a drill down which then shows who those cases are allocated to 

I want to sort this level by volume whereas it's sorted by name alpha at the moment.

How can I acheieve this?  I have data sorted by the volume measure in the table view like a similar post suggests

but it's made no difference.

Thanks for any help

Natalie