How to use last value in a custom visualization?

I have been trying to create a custom visualization with vega to display some simple data. In this case, I wanted to display the data from a vehicle where it would show the battery current vs the vehicle speed (I'm starting with something simple just to get a better understanding of how it works)

My code looks like this:

  "$schema": "",
  "title": "Battery Current vs Vehicle Speed",
  "data": {
    "url": {
      "%context%": true,
      "%timefield%": "dateTime",
      "index": "my-index",
      "body": {
        "aggs": {
          "time_buckets": {
            "date_histogram": {
              "field": "dateTime",
              "interval": {"%autointerval%": true},
              "extended_bounds": {
                "min": {"%timefilter%": "min"},
                "max": {"%timefilter%": "max"}
              "min_doc_count": 0
        "size": 0
    "format": {"property": "aggregations.time_buckets.buckets"}

  "repeat": {
    "layer": ["vehicleSpeed", "batteryCurrent"]
  "spec": {
    "mark": "line",
    "encoding": {
      "x": {
        "field": "key", 
        "type": "temporal",
        "timeUnit": "hoursminutes",
        "axis":{ title:"Date"}
      "y": {
        "aggregate": "count",
        "field": {"repeat": "layer"},
        "type": "quantitative",
        "title": false
      "color": {"datum": {"repeat": "layer"}, "type": "nominal"},
        "field": "time_buckets.buckets.key_as_string"
        "type": "temporal"
        "field": "processedData.VMU1.vehicleSpeed"
        "type": "quantitative"
        "field": processedData.batteryCurrent
        "type": "quantitative"
        "title":"Battery Current"

It renders but it doesn't have the correct values, just the two lines crossing the value 2, the Y axis is just going from 0 to 2 instead of 0 to 90 (higher value in the speed).
I know that my Y doesn't have the correct aggregation but when I change it for something else I have the error "Infinite extent for field" for both my fields, the speed and the battery current.

Any idea what could be the issue and how I can fix it?