As you can see the snapshot below:
It's quiet convenient for users to take a glance at important index, including visits, pageviews etc, but don't have the index of unique visitors, is it possible to add it in this page?
Maybe you should add your voice in #5150 (Customizable columns in All Websites Dashboard)
As @gaumondp there is already an issue about this. Apart from the mentioned one the current active issue for this is https://github.com/piwik/piwik/issues/2390 . There's also https://github.com/piwik/piwik/issues/5045