I'd like to be able to see graphs for CPU, RAM, network activity, etc. at a glance, without having to open a dedicated task monitor application.
Heads-up: this answer only applies to Loki. If anyone wants to add an answer that applies to Juno, It'd be very welcome!
You can install
sudo add-apt-repository -y ppa:indicator-multiload/stable-daily && sudo apt update && sudo apt install indicator-multiload
Then run it for the first time from the Applications menu.
Here's what it looks like, using the "Traditional" color scheme:
Kudos to SeriouslyLaughing at Reddit for the solution.
An option for Juno is the Monitor app. It has the ability to show CPU and memory usage in the wingpanel, although just that for the time being...