Email Open Activity Heat Grid report query

Hi,

I'm trying to work out the best day of the week and time of day to send emails. I've used the 'Email Open Activity Heat Grid' report which shows me this information but I would like to segment by region so that I have a better understanding. 

We use segmentation filters to segment by region but I can't see this option in the report or the option to add custom fields to the report.

I'm fairly new to Marketo so any help will be really appreciated.