Activity
Processor, memory, network and disk throughput, sampled every second and drawn beside the last ninety seconds, with the processes working hardest underneath. You can ask an app to quit from there, the way the Dock does.
Storage
What is growing right now, folder by folder, and how free space has moved over the last days.
The menu bar
Turn on the menu bar helper in Settings and free space sits in the menu bar with a ring that fills as the disk does. Its popover opens above whatever you are working in, including a full-screen app, and closes when you click away. Closing the main window then keeps DiskDuo in the menu bar rather than quitting it.