I just recreated your problem. It seems, that the graphs in Einstein Analytics just are not able to show multiple items for the same grouping.
So either you have to generate a unique grouping criteria ( e.g. AccountName + "("+AccountID+")" ) or add a second grouping to make all rows unique.
I fully agree that this is strange, but it is the correct answer: You do indeed still add the this as a derived measure in your dataset's XMD. If you have two datasets, I don't think it matters which one you add it in.
So in other words:
Click Edit for your dataset
Download the XMD file for your dataset
Modify the XMD file locally
Upload the modified XMD ...