Hi Ivo,
Putting just some random activities in the same line is not possible.
But you can achieve the goal probably by grouping the Gantt Chart. But the grouping his then switched on for the whole chart, that means for any activity, not just for some (cout annuel).
You have to assignn a value to each activity that the Grouping can work with. (All activities with the same value are grouped together) .
To get more than one bars on the same line you need to uncheck "Display Each Activity on a Separate Line"
Thorsten