Vega-lite trend line based on aggregated mark

Question

I've got a line chart in which i have a line that is aggregated by sum. I have a trend line based on regression which works fine for the none-aggregated data, but i would like it to show a trend line based on the points given by the aggregate.

here is my vega-lite specification - can someone help me out?:

  "data": {"name": "dataset"},
  "layer": [
    {
      "mark": {"type": "line"},
      "encoding": {
        "y": {
          "field": "Weekly Avg"
        },
        "color": {
          "field": "Loan Type",
          "scale": {
            "range": [
              "#4795CA",
              "#F9B96A",
              "#779E41",
              "#F54f0F",
              "#cfcfcf"
            ]
          }
        }
      }
    },
    {
      "mark": {"type": "line"},
      "encoding": {
        "y": {
          "field": "Weekly Avg",
          "aggregate": "sum"
        },
        "color": {"datum": "Total"}
      }
    },
    {
      "mark": {
        "type": "line",
        "strokeDash": [5, 5]
      },
      "transform": [
        {
          "regression": "Weekly Avg",
          "on": "W / C",
          "groupBy": "W / C"
        
        }
      ],
      "encoding": {
        "y": {
          "field": "Weekly Avg"
        },
        "color": {"datum": "Trend"}
      }
    }
  ],
  "encoding": {
    "x": {
      "field": "W / C",
      "timeUnit": "monthdateyear",
      "type": "ordinal",
      "axis": {"format": "%d/%m/%Y"}
    },
    "y": {
      "type": "quantitative",
      "axis": {"format": "~s"}
    }
  }
}