hi @Haritz-Lopez I think this is not possible at the moment in an easy way, of course, you can always use the xlsx transformation and that should allow you to do anything you want, but it is going to be more complex of course and also it will require you to spend time understanding the changes you need to make in the internal xml files of the xlsx.
i am working right now on having support for multi-rows each support, when that is implemented it should be possible to achieve this. basically, you will make an iteration that involves multiple rows, and those rows are going to be repeated based on that data.
Is this possible to do directly with jsreport or should I first, somehow, prepare the data and give everything "done" to jsreport?
you can also prepare the data inside the jsreport with a script or helpers, so you can transform it and group it based on your needs, but as described above you are going to need to have the muti-rows each to be supported in order to have this working easily for you, i am working on it.