Show harddisk activity in Ubuntu 20.04

I have Ubuntu 20.04 and I use system monitor to see cpu and ram activity but it doesn't show the harddisk activity like the ram utilization is showed in graph and its reading and writing speed so how to enable it?

The gnome shell extension called gnome-shell-extension-system-monitor can put a real-time graph into the top panel.

You need to install the Gnome Tweaks app along with the extension:

sudo apt install gnome-tweaks gnome-shell-extension-system-monitor

The newly installed Tweaks app will offer a user interface for managing shell extensions.

Shell extensions need to be enabled in general (look for the "Extensions" tab in the Tweaks app window), and then the "System Monitor" can be enabled in the list of extensions.

It's highly configurable. Maybe the "Disks" graph is not enabled by default, but via the preferences, it can be enabled.

To take full effect, might need logging out and back in.