Thanks for your answer, this cleared some things for me.

Why do you want to define the model as a QAbstractItemModel and not as a QStandardItemModel?
Well, this was not my idea I grabbed some code from Qt's build-in examples, this one was from itemviews/chart/mainwindow.h and mainwindow.cpp.

Let the model add the indexes for you and use appendItem or insertItem.
This is done with QModelIndex(), right? I have to read about that first. Does a model assign an QModelIndex to its items automatically or do I have to create one?

Kind regards,
HomeR