Where do you define how a statistics report that shows up in email reports is displayed?

I created a custom report which basically consists of a DataTable filled with some custom data.
It shows up perfectly when I visit the report on the piwik site but when I add it to the email report the table is completely different (2 columns showing only 1 actual row vs 5 columns with 10 rows on the original report).
It also seems to add actions/visit, bounce rate, etc automatically while these columns do not exist in the report I created myself.
What defines how a report that is added to an email report is made up? I’ve gone through a lot of code in the existing plugins but I can’t for the life of me find it anywhere…

Some more details about my current plugin:
Created a custom report for the plugin, only modified the metrics array and added some extra columns.
Added the method that returns the DataTable to the API that I added to the plugin, this one is filled with some manual query data.
Added some functionality to the main plugin class.
After this I have a fully functional custom tracker under the Visitors subsection.