Combine XYPlot and Scatter Plot

It's possible with a custom visual called Deneb - - but it takes a bit of learning.  

More about Deneb here:

Deneb - Declarative Visualization in Power BI (deneb-viz.github.io)

{
  "data": {"name": "dataset"},
    "layer": [
      {
        "mark": {
          "type": "point",
          "opacity": 0.5
        },
        "encoding": {
          "x": {
            "field": "Sale Price",
            "type": "quantitative"
          },
          "y": {
            "field": "Units",
            "type": "quantitative"
          },
          "color": {
            "field": "Project",
            "type": "nominal"
          }
        }
      },
      {
        "mark": {"type": "line"},
        "encoding": {
          "x": {
            "field": "Sale Price",
            "type": "quantitative"
          },
          "y": {
            "aggregate": "mean",
            "field": "Units",
            "type": "quantitative"
          },
          "color": {
            "field": "Project",
            "type": "nominal"
          }
        }
      }
    ]
}