The first thing that you need to do when creating your own Custom Report is to select the different widgets that you would like displayed. As you might already know, you can add the widgets that suit you best by choosing from the different widget categories available and then explore their amazing potential.
Τhe Mailing Lists Category is divided into two subcategories for you to choose widgets from: Mailing List that includes information regarding your email list performance and Members that includes information regarding your members' activity.
In the Mailing List subcategory, one of the widgets included is named "Engagement Breakdown". This widget displays a chart comparing your members' engagement based on their total campaign opens. Please note that said widget can be filtered by Mailing list.
1. Select the Engagement Breakdown widget by clicking on the Add button and then tap on Save. 
Your Custom Report has already been filtered by the most recently created mailing list, therefore this filter will be applied to the widget selected as well.
2. In case you want to change the selected mailing list, tap on the corresponding field at the top of the widget's frame. A drop-down menu with all your mailing lists appears, allowing you to select the list by which you want your widget filtered.
Please keep in mind that there are several filters to choose from that can be applied to the corresponding widgets. Furthermore, you can always export the data of each widget in order to have them at your disposal at all times.