When I created an account, it was possible to choose the free option not only for students, but also "home", only there is a limited number of channels to create, 3 I think. There is also a limited number of data logins, but for these 3 channels, each with 8 fields, every 15 minutes is enough.
Otherwise, you can install your own thinkspeak, e.g. if you have a Synology or Qnap network drive, install the docker and there are ready images from thinkspeak. All you need is an external IP, if you want to connect to such a server, e.g. from your phone to generate such a graph using a widget on the phone desktop anywhere in the world. You can also replace the network drive with an RPi, a terminal for a couple of bucks from Allegro or an old laptop. I have also run such a server on a tablet, the hardest thing was to replace Windows with Linux, once there is Linux it is a tutorial from the internet and step by step docker, thinkspeak image selection and it works.
Helpful post? Buy me a coffee.