Pivot Table Not Sorting "Wk_1" through "Wk_52" Correctly

I am building a YoY sales graph for work and have been asked to use the format "Wk_1", "Wk_2", etc. as my charting values. I build the graph off of a pivot table since I do not need all of 2018 sales - just those from Wk_1 through last week 2019 (Wk_37) (since this is a weekly chart, I will cap the pivot at Week 37 and next week extend to 38, 39, etc.).

However, my weeks are sorting alphabetically as "Wk_1", "Wk_10", "Wk_11", etc. I get this is a string, but there's some way to arrange these in order since I've seen charts at my company with this formatting. We recently switched from Office 2010 to 2016, so maybe the process is different. If anyone can get it to work in the attached and advise how, I'd be grateful!

Thanks, this was approved! Still not sure why it does not work for me the other way, though, since it used to.