This may not be possible in Analytics. There is a module called RCA that has more ad hoc features such as trending over time that can handle these issues. Not sure if you have it, it is an upgrade.
Thanks for the response. Fortunately, we do have RCA and I have been able to build an email analysis report from it that provides me what I need. Unfortunately, I was hoping there would still be a way to accomplish through Analytics since one of the companies we recently acquired (who currently runs their own instance of Marketo) does not have RCA. We have been trying to consolidate our reporting and until we merge our 2 Marketo instances together, I was hoping we could provide an adequate interim solution for them using Analytics.