Mailchimp notes/limitations

MailChimp’s API only allows reporting software to automatically retrieve data in real time when requested. No historical data can be retrieved.
The metric "Unsubscribe Count" is only reporting data for audience lists, not for specific campaigns (even if the campaign has a very similar name compared to the audience list). Be mindful of this detail when reporting this to your client.
When creating a widget in DashThis with MailChimp's integration, you will see cumulative data attributed to your campaigns. But you won't be able to see the specific results associated with past periods.
In order to have the most accurate data collected for a particular campaign, we recommend you create a new widget as soon as you launch your campaign, and filter it by its campaign name. This will allow you to monitor a specific metric for that particular campaign.