Only display legend on filtered data

Hi there

I have a line graph which shows total invoices processed over time. I don't want a legend on it, so it's just gone a single line. However when someone filters the data by employee, I want the data to be split by Cost Area. 

I can put Cost Area as the legend, but it filters it obviously when there is no selection.

I'm not sure this is even possible, but if it is, one you guys will know!

Jemma