The bad news is that WK doesn’t provide any way of getting this historical data, so you can only get a graph starting from today. (That’s why my graph starts at level 5 – that’s when I started collecting data.)
The good news is that it can keep itself up to date automagically, thanks to Google Sheets. Original post and instructions here: WaniKani API + Google Docs Awesomeness
If you like how mine looks you can copy it from here: WaniKani data (template)