If you had a linux VPS, which didn't come with any graphs showing things like processor, RAM, disk usage, etc, what (hopefully free) thing would you use to monitor all that?

I'm used to using Heroku but am trying out a VPS that doesn't come with any graphing and it feels very opaque – I'd like to see if I'm approaching its limits! I've done some googling and most options seem very complicated, require a separate server to run the graphs on, or are expensive enterprise solutions. Or all three.

What's simple and maybe free?
Can you SSH into your sever? If you can, give it the ole 

htop

That's assuming your server has htop installed, which it probably does. Might depend on your provider, I guess

httpshtop.dev/