@fcarney I put the QAbstractItemModelTester in both my model, and my proxy model, and... no errors. I also swapped it to FailureReportingMode::Fatal to double check, and nothing. The endless calls to data from VXYModelMapper continue.

@Chris-Kawa Thank you very much, this worked and was exactly what I needed to resolve my issue. Thank you also for the explanations, clear and precise.

Edit: I have checked that if coordinates are [0.3 0.5 0.3 0.9] and I set pen.widthf bigger, then I can select much better the lines, but if the coordinates are [30 50 30 90] that does not work...

@mrjj hi my friend. You helped me a lot. That works fine!!!!!!! Superb!!!!!!! Thanks again