Hi @Justin365,
I was thinking through this and did some more research and have some additional thoughts.
1. If the Marketing Lists were "Static" vs. "Dynamic" from a membership perspective there is already a "Member Count" field on the Marketing List table that would be populated with the total members in the list.

If you have this you can create a chart in the Marketing List entity like the following:

From here you can create your own dashboard like the following adding this chart:

Let me know if this is what you are looking for... If your Marketing Lists are Dynamic in membership well, then we have another discussion as some of the steps above will not work because the counts are not populated for Dynamic Marketing Lists...
Hope this helps. Please mark accepted if solved your question and Like if helped at all.
Thanks much,
Drew