Thanks for the quick reply!
With that method, I can now indeed change the dateformat to weekyear instead of just week, so that is great!
However the chart then shows all values, also week 44 2014.
Can I change the chart to showing only week 44 2015 until week 30 2016, while the other values are still taken into account for the calculation (so filtering doesn't work)?
I also see that I made a mistake in my formula and the same is happening in the suggestion above.
It is not calculating the YoY difference but rather the difference from week 1 to week 2 etc.
Any suggestions on how to fix this?