Hi Thomas, the comment above was from me.
Yes, each group node was converted into a swimlane node with multiple columns.
When I import my test data, I get 4 groups (Q1-4), with the months in the correct group. January is out on its own (correct). The edges are correct.
When I select the group nodes, and apply the swimlane node shape, I get the following:
4 swimlane nodes, each with 2 columns. Jan is still on its own. The other months are in the columns of each group swimlane. I can't tell why the swimlanes have two columns.
It doesn't matter though - the behavior is correct.
As you say below, it can't be done at this time. I think I will up-vote the feature request though.