In the on-prem version of Charts the page at
https://docs.mongodb.com/charts/onprem/data-sources#std-label-add-data-source

mentions,

Procedure
From the Data Sources list, click the New Data Source button.
Enter the Connection String URI of the MongoDB deployment to which you want to connect.

For the Atlas version I would like to pull data from any of my Internet accessible MongoDB servers to form a more complete enterprise dashboard. Much like with Compass, all that is needed is a connection string and you are off too charting. The strict only Atlas data access is a bit too restrictive IMHO.