It's weird how people seem to spin their nodes down. If you have the hardware already, it doesn't cost much to run.
I would expect steady growth over time, but it doesn't look great from an adoption point of view.
Discussion
most people look at wrong data always tell me there are only 20k nodes. do you got a link to your data?
I found this link to Luke dash's site, I think he aggregates the node data in a different way since the 20k number is solely publicly accessible nodes.
https://luke.dashjr.org/programs/bitcoin/files/charts/historical.html