CALCULATE Function Filter Help

Hello,

I am new to Power BI and have done some basic training through Microsoft but need some help-

I want to build a line graph that has 2 lines:

1) Total performance over time for a catagory of products, all customers

2) Total performance over time for a catagory of products, a specific customer- with this being changed by a slicer.

The aim is that my sales team can see how different retailers are doing with a product compared to the market as a whole.

I have determined that I need to somehow have the "customer" slicer to not impact line 1 referenced above.

I beleive I can do this using a CALCULATE function but have spent the best part of a day looking online and trying myself- any help would be really appreciated!

Please let me know if there is anything else/screenshots that I can provide to help-

Thanks,

Francesca