How do you create a 2 series vega chart from 2 not nested aggregations

I'm trying to create a 2 series vega chart from the following sample response. I have 2 date_histogram aggregations and I want to plot the doc count for each month. Should I be using a format/transform on the data or should I be using layers? I can't figure out how to extend the sample when you create a new vega visualization.

{
  "aggregations" : {
    "dh_created" : {
      "buckets" : [
        {
          "key_as_string" : "2019-03-01T00:00:00.000Z",
          "key" : 1551398400000,
          "doc_count" : 29173142
        }
      ]
    },
    "dh_startTime" : {
      "buckets" : [
        {
          "key_as_string" : "2019-03-01T00:00:00.000Z",
          "key" : 1551398400000,
          "doc_count" : 41044891
        }
      ]
    }
  }
}

Hi @TealFawn
Sorry for the delay, have you already found a solution to your problem? if not I'm more then happy to can take a look and give you an hint

I never found a solution but I have moved on from the issue now.