There's another way to power up your Data Studio reporting with outside data sources like Mailchimp.  

Enter Google Apps Script.  It takes a bit of programming knowledge, but this template allows you to get up and running without writing any code.

It pulls your Mailchimp campaign and list growth stats into Sheets, and includes a Data Studio template to visualize your results.  

Take the template for a test drive here:

https://docs.google.com/spreadsheets/d/1evL6Lg0gqMO8zVOFWCcHtgo9-1ifmWs7JxWDQush3Tc/copy