Describe Welsh studies of learning Curves in Second Language Acquisition

Dear writer, my PhD supervisor has just been to a Conference at Wales University. The presenter, Ennli M. Thomas from Bangor University, Wales, was showing some slides with non-linear graphs of Welsh grammar learning. Here is the link to this professor: https://www.bangor.ac.uk/education-and-human-development/Staffpages/EnlliThomas.php.en Kindly can you refer to any sources/publications about these graphs of Welsh grammar acquisition?