Show mailing list subscriptions total number and monthly increase

[ilascar]

Problem:
I would like to know how many people are following news and updates from Designers Italia and how much this number is increasing in time.

Solution:
Adding a tab dedicated to total number of subscriptions (format similar to the tab 'Utenti slack developers')
Adding a tab showing in % the increase for the past month

Alternatives:
Also other visualization and diagrams types is fine, it would be nice to show distribution at regional level but I don't think this data is available.