Separating MailChimp data by month?

jessem - Im not sure I completely understand your question but I will answer the best I can. If you want to split the dates by month you can try this option (its what I do). In excel create a table with dates you need, for example from 01/01/2014 - 31/12/2015. In the column next to it copy and paste the same information again but change the formatting to be custom/type M. This will give you the month as a number. Repeat this step again in another column and change formatting to be custom/type MMM. This will give you the month like this Jan, Feb etc.

Now with the new create table feature in power BI you can copy this data into PBI. The reason you need a column with the number of the month is so you can sort the column with months as text (Jan, Feb, etc). Click on the column with the worded months and then click the sort by key in the ribbon, and sort my number month. Now the worded months will always be in the correct order.

Once this is done create a relationship between you MailChimp data and the date table through the dates. Now you will be able to filter your data by month. 

I hope this helps. It takes a bit of prework, but once it is done it makes sorting by month easy. You can then start adding in other date fields such as financial year/week/month, quarter, etc.