Retrieve data from nested JSON to Morris.js xkey.

So I'm trying to make a bar chart, with morris.js, using CakePHP 3. I have the following nested JSON,(wich contains date details):

MarketingSales:
 0: Object
 country_id: 5
 date: Object
   NumeroDia: 4
   date: "2015-01-01 T 00:00:00+0000"
   day: 1
   id: 366
   month: 1
   week: 1
   year: 2015
 date_id: 366
 id: 4106
 medium_id: 8
 sales: 54189.18863
 views: 3496

This JSON comes from two joined tables ("MarketingSales" and "Dates"), using the "contains" option from CakePHP.

I'm now trying to label the x axis with the number of the month, but I'm getting "undefined" labels.

so far, I have tried:

xkey:['dates']
xkey:['dates.month']
xkey:['dates']['month']
xkey:dates.month

Reading through morris.js documentation I found about setting parseTime to false, but it doesn't work either.

Any ideas of what I'm doing wrong?

Thanks!