Joining multiple layers on more than one column is possible in Chartio. We'll illustrate this with an example.
In our chart, we want to group our Payments by Date and then by Plan Cost. We want to separate our plans into two layers: Low Cost and High Cost, which we'll accomplish by filtering the layers by the Cost column. Here's what our first layer looks like:
We'll then need to join our layers on the first two columns. We'll select Outer Join as the merge type, and select 2 from the Join on first N columns dropdown. This allows us to join our layers on both the Month of Payment Date and Cost columns.
I've also checked Show row totals for numeric columns in the Chart Settings so I can compare between the Low and High cost totals. Here's what our final chart looks like:
For more details on joining layers, please see our documentation on Merging Data.