The tension graph already shows that the Y axis has problems.
What happened: I had a broken belt on X0 and replaced is. In the process I had to undo all belts and obviously put them back, tensioned them as equally as I could to get the gantry square which works out quite OK on X. The bets are tensioned to 82Hz at 20 cm (or 12 N tension) on both X and Y. I used to have poor X and much better though definitely not perfect Y axis.