[enhancement] disk activity

[mattia-b89]

New disk activity is great;
anyway, the indicator, is always green (maybe it points to disk usage rather than disk activity?!)
making it not useful, as it should be:

• disk activity changes rapidly over time
• disk usage does not change rapidly over time

I share a screenshot showing that upper indicator is fully green while chart is almost empty:

[fflewddur]

Yes, it shows filesystem usage, and this is by design. In the future I'd like to also support disk activity, similar to the network monitor.