The criteria for placing any item in a different row from the previous one is the overlapping. If the RadTimeline control detects that two items are overlapping (even with fraction of the second) it will place one of the items on different row. This criteria does not change when there is grouping.
Regarding your question about controlling the positioning of the items, the answer is yes. You can define custom row index generator class. In it you will be able to define custom logic based on which the items will be positioned. You can take a look at our online demo
to see a sample implementation.
Regarding the requirement to hide the "+" sign of the groups, you can achieve this by setting the GroupExpandMode property of the RadTimeline to None. This will prevent the groups from collapsing and remove the sign.
I hope this information is helpful.
Pavel R. Pavlov
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