At the moment it looks as though the Comparison feature does not work on the All Websites dashboard.

1. When going to the All Websites dashboard through the top menu it does not show an option to do a comparison when viewing the All Websites dashboard from here.
   

2. When going to the All Websites dashboard through the Site Selector the option to compare shows, but when selecting a period to compare to there is no comparison data shown in the report.
   
   
   (Screenshots were taken from demo.matomo.cloud)

We can either remove this option from showing on the All Websites dashboard when navigating to it from the Site Selector, or we can implement the feature to work on the All Websites dashboard.